drm/amdgpu: enable mes to access registers v2

Enable mes to access registers.

v2: squash mes sched ring enablement flag

Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>