drm/amdgpu/vce1: Workaround PLL timeout on FirePro W9000

drm/amdgpu/vce1: Workaround PLL timeout on FirePro W9000

Sometimes the VCE PLL times out waiting for CTLACK/CTLACK2.
When it happens, the VCE still works, but much slower.
Observed on a Tahiti GPU, but not all:
- FirePro W9000 has the issue
- Radeon R9 280X not affected
- Radeon HD 7990 not affected

As a workaround, on the affected chip just don't put the
VCE PLL in sleep mode. Leaving the VCE PLL in bypass mode
or reset mode both work. Using bypass mode is simpler.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>