drm/radeon: add UVD support for older asics v4

v2: cleanup R600 support
v3: rebased on current drm-fixes-3.12
v4: rebased on drm-next-3.14

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