drm/xe: Allow const propagation in gt_to_xe()

Replace the inline function with a _Generic() so gt_to_xe() can work
with a const struct xe_gt*, which leads to a const struct xe *.
This allows a const gt being passed around and when the xe device is
needed, compiler won't issue a warning that calling gt_to_xe() would
discard the const. Rather, just propagate the const to the xe pointer
being returned.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>