drm/xe/gt_idle: Convert register access to use xe_mmio

Stop using GT pointers for register access.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240910234719.3335472-84-matthew.d.roper@intel.com