drm/radeon: restructure UVD code to handle UVD PG (v2)

When we PG (powergate) UVD, we need to re-initialize it
before we can use it again.

v2: rebase on UVD stop fixes

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

diff --cc drivers/gpu/drm/radeon/cik.c
Simple merge

diff --cc drivers/gpu/drm/radeon/evergreen.c
Simple merge

diff --cc drivers/gpu/drm/radeon/ni.c
Simple merge

diff --cc drivers/gpu/drm/radeon/r600.c
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon_asic.h
Simple merge

diff --cc drivers/gpu/drm/radeon/rv770.c
Simple merge

diff --cc drivers/gpu/drm/radeon/si.c
Simple merge