drm/amdgpu: Remove check which is not valid for certain VBIOS

Fixes vbios fetching on certain headless boards.

Signed-off-by: Ken Wang <Ken.Wang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org