drm/xe/pat: trim the xelp PAT table

We don't seem to use the 4-7 pat indexes, even though they are defined
by the HW. In a future patch userspace will be able to directly set the
pat_index as part of vm_bind and we don't want to allow setting 4-7.
Simplest is to just ignore them here.

Suggested-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Pallavi Mishra <pallavi.mishra@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>