drm/i915/gt: Pass gt rather than uncore to lowest-level reads/writes

Passing the GT rather than uncore to the lowest level MCR read and write
functions will make it easier to introduce dedicated MCR locking in a
following patch.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221128233014.4000136-3-matthew.d.roper@intel.com