drm/xe/mtl: Disable media GT

drm/xe/mtl: Disable media GT

Xe incorrectly conflates the concept of 'tile' and 'GT.'  Since MTL's
media support is not yet functioning properly, let's just disable it
completely for now while we fix the fundamental driver design.  Support
for media GTs on platforms like MTL will be re-added later.

v2:
 - Drop some unrelated code cleanup that didn't belong in this patch.
   (Lucas)
v3:
 - Drop unnecessary xe_gt.h include.  (Gustavo)

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Gustavo Sousa <gustavo.sousa@intel.com>
Reviewed-by: Matt Atwood <matthew.s.atwood@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Acked-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://lore.kernel.org/r/20230601215244.678611-2-matthew.d.roper@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>