drm/radeon/cik: fill in startup/shutdown callbacks (v5)

v2: update to latest driver changes
v3: properly tear down vm on suspend
v4: fix up irq init ordering
v5: remove outdated comment

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