drm/amdgpu/uvd7: add emit_reg_write_reg_wait ring callback

This adds support for writing and reading back using the
helper since the engines doesn't have a oneshot packet.

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