drm/xe/uapi: Kill tile_mask

It is currently unused, so by the rules it cannot go upstream.
Also there was the desire to convert that to align with the
engine_class_instance selection, but the consensus on that one
is to remain with the global gt_id. So we are keeping the gt_id
there, not converting to a generic sched_group and also killing
this tile_mask and only using the default behavior of 0 that is
to create a mapping / page_table entry on every tile, similar
to what i915.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Francois Dugast <francois.dugast@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>