drm/xe: Map both mem.kernel_bb_pool and usm.bb_pool

For integrated devices we need to map both mem.kernel_bb_pool and
usm.bb_pool to be able to run batches from both pools.

Fixes: a682b6a4

 ("drm/xe: Support device page faults on integrated platforms")
Tested-by: Brian Welty <brian.welty@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Brian Welty <brian.welty@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240202033440.2351862-1-matthew.brost@intel.com