drm/xe/pcode: Treat pcode as per-tile rather than per-GT

There's only one instance of the pcode per tile, and for GT-related
accesses both the primary and media GT share the same register
interface.  Since Xe was using per-GT locking, the pcode mutex wasn't
actually protecting everything that it should since concurrent accesses
related to a tile's primary GT and media GT were possible.

Fixes: dd08ebf6

 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240829220619.789159-5-matthew.d.roper@intel.com
(cherry picked from commit 3034cc8107b8d0c7d1b56584394e215dab57f8a3)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>