drm/amd/display: Fix NULL pointer dereference for DTN log in DCN401
In the Linux kernel, the following vulnerability has been resolved:

drm/amd/display: Fix NULL pointer dereference for DTN log in DCN401

When users run the command:

cat /sys/kernel/debug/dri/0/amdgpu_dm_dtn_log

The following NULL pointer dereference happens:

[  +0.000003] BUG: kernel NULL pointer dereference, address: NULL
[  +0.000005] #PF: supervisor instruction fetch in kernel mode
[  +0.000002] #PF: error_code(0x0010) - not-present page
[  +0.000002] PGD 0 P4D 0
[  +0.000004] Oops: 0010 [#1] PREEMPT SMP NOPTI
[  +0.000003] RIP: 0010:0x0
[  +0.000008] Code: Unable to access opcode bytes at 0xffffffffffffffd6.
[...]
[  +0.000002] PKRU: 55555554
[  +0.000002] Call Trace:
[  +0.000002]  <TASK>
[  +0.000003]  ? show_regs+0x65/0x70
[  +0.000006]  ? __die+0x24/0x70
[  +0.000004]  ? page_fault_oops+0x160/0x470
[  +0.000006]  ? do_user_addr_fault+0x2b5/0x690
[  +0.000003]  ? prb_read_valid+0x1c/0x30
[  +0.000005]  ? exc_page_fault+0x8c/0x1a0
[  +0.000005]  ? asm_exc_page_fault+0x27/0x30
[  +0.000012]  dcn10_log_color_state+0xf9/0x510 [amdgpu]
[  +0.000306]  ? srso_alias_return_thunk+0x5/0xfbef5
[  +0.000003]  ? vsnprintf+0x2fb/0x600
[  +0.000009]  dcn10_log_hw_state+0xfd0/0xfe0 [amdgpu]
[  +0.000218]  ? __mod_memcg_lruvec_state+0xe8/0x170
[  +0.000008]  ? srso_alias_return_thunk+0x5/0xfbef5
[  +0.000002]  ? debug_smp_processor_id+0x17/0x20
[  +0.000003]  ? srso_alias_return_thunk+0x5/0xfbef5
[  +0.000002]  ? srso_alias_return_thunk+0x5/0xfbef5
[  +0.000002]  ? set_ptes.isra.0+0x2b/0x90
[  +0.000004]  ? srso_alias_return_thunk+0x5/0xfbef5
[  +0.000002]  ? _raw_spin_unlock+0x19/0x40
[  +0.000004]  ? srso_alias_return_thunk+0x5/0xfbef5
[  +0.000002]  ? do_anonymous_page+0x337/0x700
[  +0.000004]  dtn_log_read+0x82/0x120 [amdgpu]
[  +0.000207]  full_proxy_read+0x66/0x90
[  +0.000007]  vfs_read+0xb0/0x340
[  +0.000005]  ? __count_memcg_events+0x79/0xe0
[  +0.000002]  ? srso_alias_return_thunk+0x5/0xfbef5
[  +0.000003]  ? count_memcg_events.constprop.0+0x1e/0x40
[  +0.000003]  ? handle_mm_fault+0xb2/0x370
[  +0.000003]  ksys_read+0x6b/0xf0
[  +0.000004]  __x64_sys_read+0x19/0x20
[  +0.000003]  do_syscall_64+0x60/0x130
[  +0.000004]  entry_SYSCALL_64_after_hwframe+0x6e/0x76
[  +0.000003] RIP: 0033:0x7fdf32f147e2
[...]

This error happens when the color log tries to read the gamut remap
information from DCN401 which is not initialized in the dcn401_dpp_funcs
which leads to a null pointer dereference. This commit addresses this
issue by adding a proper guard to access the gamut_remap callback in
case the specific ASIC did not implement this function.