drm/radeon: Changing number of compute pipe lines

The current CP firmware can handle Usermode Queues only on MEC1.
To reflect this firmware change, this commit reduces number of compute pipelines
to 4 - 1, from 8 - 1 (the first pipeline is allocated for kgd).

Signed-off-by: Ben Goz <ben.goz@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>
Cc: stable@vger.kernel.org