drm/amdgpu: Check if FB BAR is enabled for ROM read

Some configurations don't have FB BAR enabled. Avoid reading ROM image
from FB BAR region in such cases.

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>