drm/amdgpu/vce: Clear VCPU BO, don't unmap/unreserve (v4)

drm/amdgpu/vce: Clear VCPU BO, don't unmap/unreserve (v4)

The VCPU BO doesn't only contain the VCE firmware but also other
ranges that the VCE uses for its stack and data. Let's initialize
this to zero to avoid having garbage in the VCPU BO.

Additionally, don't unmap/unreserve the VCPU BO.
The VCPU BO needs to stay at the same location before and after
sleep/resume because the FW code is not relocatable once it's
started.

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