drm/radeon/kms: avoid oops on mac r4xx cards

They don't have an atombios so don't attempt to use it for
eng/mem clocks.

Reported by spoonb on #radeon
fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=28671

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>