drm/radeon: restructure cg/pg on cik (v2)

- use new cg/pg flags for finer grained clock and
powergating control
- restructure the cg/pg code so it can be called from
other components such as dpm

v2: fix build breakage from rebase

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>