drm/radeon/kms: enable UVD as needed (v9)

When using UVD, the driver must switch to a special UVD power
state.  In the CS ioctl, switch to the power state and schedule
work to change the power state back, when the work comes up,
check if uvd is still busy and if not, switch back to the user
state, otherwise, reschedule the work.

Note:  We really need some better way to decide when to
switch out of the uvd power state.  Switching power states
while playback is active make uvd angry.

V2: fix locking.

V3: switch from timer to delayed work

V4: check fence driver for UVD jobs, reduce timeout to
    1 second and rearm timeout on activity

v5: rebase on new dpm tree

v6: rebase on interim uvd on demand changes

v7: fix UVD when DPM is disabled

v8: unify non-DPM and DPM UVD handling

v9: remove leftover idle work struct

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Christian König <deathsimple@vodafone.de>