drm/radeon/kms: dynclks fixes

- only r4xx/r5xx/rs6xx/rs740 have clock gating atom table,
so disable it on r6xx.  it's already disabled on r7xx
- check to make sure the clock_gating hook exists before
calling it.  This avoids a segfault on asics without
that function.
- remove unused static power management function.

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