drm/amdgpu: Avoid fetching vram vendor information

For GFX 9.4.3 APUs, the current method of fetching vram vendor
information is not reliable. Avoid fetching the information.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>