drm/radeon/kms: add dpm support for rv6xx (v3)

This adds dpm support for rv6xx asics.  This includes:
- clockgating
- dynamic engine clock scaling
- dynamic memory clock scaling
- dynamic voltage scaling
- dynamic pcie gen1/gen2 switching

Set radeon.dpm=1 to enable.

v2: remove duplicate line
v3: fix thermal interrupt check noticed by Jerome

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>