drm/xe: Include hardware prefetch buffer in batchbuffer allocations

drm/xe: Include hardware prefetch buffer in batchbuffer allocations

The hardware prefetches several cachelines of data from batchbuffers
before they are parsed.  This prefetching only stops when the parser
encounters an MI_BATCH_BUFFER_END instruction (or a nested
MI_BATCH_BUFFER_START), so we must ensure that there is enough padding
at the end of the batchbuffer to prevent the prefetcher from running
past the end of the allocation and potentially faulting.

Bspec: 45717
Link: https://lore.kernel.org/r/20230329173334.4015124-2-matthew.d.roper@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>