drm/amdgpu: init return value in amdgpu_ttm_clear_buffer
In the Linux kernel, the following vulnerability has been resolved:

drm/amdgpu: init return value in amdgpu_ttm_clear_buffer

Otherwise an uninitialized value can be returned if
amdgpu_res_cleared returns true for all regions.

Possibly closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3812

(cherry picked from commit 7c62aacc3b452f73a1284198c81551035fac6d71)