drm/radeon: separate UVD code v3

Our different hardware blocks are actually completely
separated, so it doesn't make much sense any more to
structure the code by pure chipset generations.

Start restructuring the code by separating our the UVD block.

v2: updated commit message
v3: rebased and restructurized start/stop functions for kv dpm.

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