drm/radeon/kms: more pm fixes

- disable gui idle interrupt use
  Seems to hang some r5xx chips
- move vbl range check into
  existing vbl check function in
  radeon_pm.c
- disable crtc mc acccess for the
  whole reclocking process

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