drm/xe: Create dedicated xe_mmio structure

Pull the 'mmio' substructure from xe_tile out into a dedicated type.
Future patches will expand this structure and then eventually move MMIO
read/write operations over to using this type.

v2:
 - Fix kerneldoc of 'size' field.  The rename/refocusing of this field
   got moved to the next patch of the series.  (Lucas)
 - Correct commit message; it's the tile, not the device, mmio that's
   been pulled out to a separate type.  (Michal)

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240910234719.3335472-47-matthew.d.roper@intel.com