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

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: Add null check for 'afb' in amdgpu_dm_update_cursor (v2) This commit adds a null check for the 'afb' variable in the amdgpu_dm_update_cursor function. Previously, 'afb' was assumed to be null at line 8388, 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.c:8433 amdgpu_dm_update_cursor() error: we previously assumed 'afb' could be null (see line 8388)