drm/radeon: Changing number of compute pipe lines

commit e405ca3a upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>