drm/amdgpu: correct cp doorbell range

1. move MES doorbell inside the mec doorbell range,
   for mes belongs to mec block
2. setting the correct gfx/mec doorbell range, so that
   fw can correctly detect gfx/compute work load to enter/exit
   power saving state.

Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Tested-and-acked-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>