drm/radeon/kms: R600/RV770 remove dead code and print message for wrong BIOS

R600 & RV770 family are all using atombios so remove dead code and
print an error message if we fail to find a valid atombios.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>