drm/amd/display: Pass non-null to dcn20_validate_apply_pipe_split_flags
In the Linux kernel, the following vulnerability has been resolved:

drm/amd/display: Pass non-null to dcn20_validate_apply_pipe_split_flags

[WHAT & HOW]
"dcn20_validate_apply_pipe_split_flags" dereferences merge, and thus it
cannot be a null pointer. Let's pass a valid pointer to avoid null
dereference.

This fixes 2 FORWARD_NULL issues reported by Coverity.