drm/radeon: fix halting UVD

Removing the clock/power or resetting the VCPU can cause
hangs if that happens in the middle of a register write.

Stall the memory and register bus before putting the VCPU
into reset. Keep it in reset when unloading the module or
suspending.

Signed-off-by: Christian König <christian.koenig@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>