drm/amd/display: Add null check for 'afb' in amdgpu_dm_plane_handle_cursor_update (v2)

In the Linux kernel, the following vulnerability has been resolved:

drm/amd/display: Add null check for 'afb' in amdgpu_dm_plane_handle_cursor_update (v2)

This commit adds a null check for the 'afb' variable in the

amdgpu_dm_plane_handle_cursor_update function. Previously, 'afb' was

assumed to be null, but was used later in the code without a null check.

This could potentially lead to a null pointer dereference.

Changes since v1:

- Moved the null check for 'afb' to the line where 'afb' is used. (Alex)

Fixes the below:

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_plane.c:1298 amdgpu_dm_plane_handle_cursor_update() error: we previously assumed 'afb' could be null (see line 1252)