drm/xe: Move forcewake to 'gt.pm' substructure

Forcewake is a general GT power management concept that isn't specific
to MMIO register access.  Move the forcewake information for a GT out of
the 'mmio' substruct and into a 'pm' substruct.  Also use the gt_to_fw()
helper in a few more places where it was being open-coded.

v2:
 - Kerneldoc tweaks.  (Lucas)

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/20240910234719.3335472-46-matthew.d.roper@intel.com