drm/radeon: don't disable plls that are in use by other crtcs

commit 4e58591c8961b3e31709313f75819f2eec06e322 upstream.

Some plls are shared for DP.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
[bwh: Backported to 3.2: add the dev and rdev variables, previously added
 upstream to support DCE6.1 which isn't supported in this version]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>