drm/radeon/kms: Test rdev->bios centrally in combios_get_table_offset().

It's happened time and time again (most recently with the support for EDID
hardcoded in the BIOS ROM) that new code didn't check for rdev->bios being
non-NULL before triggering dereferences of it. This would result in an
oops/panic on setups with no BIOS ROM. Hopefully this central test will be
more robust.

Signed-off-by: Michel Dänzer <daenzer@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>