drm/radeon: implement UVD hw workarounds for R6xx v3

Only the essentials, cause this hw generation is really buggy.

v2: start supporting RV670,RV620 and RV635 as well
v3: activate more workarounds

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