drm/xe: Let primary and media GT share a kernel_bb_pool

The media GT requires a valid gt->kernel_bb_pool during driver probe to
allocate the WA and NOOP batchbuffers used to record default context
images.  Dynamically allocate the bb_pools so that the primary and media
GT can use the same pool during driver init.

The media GT still shouldn't be need the USM pool, so only hook up the
kernel_bb_pool for now.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20230410200229.2726648-1-matthew.d.roper@intel.com


Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>