drm/xe: Clarify GT counting logic

drm/xe: Clarify GT counting logic

The total number of GTs supported by a platform should be one primary
GT, one media GT (if media version >= 13), and a number of remote tile
GTs dependent on the graphics IP present.  Express this more clearly in
the device setup.

Note that xe->info.tile_count is inaccurately named; the rest of the
driver treats this as the GT count, not just the tile count.  This
will need to be cleaned up at some point down the road.

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20230406235621.1914492-6-matthew.d.roper@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>