drm/i915/mtl: fix mocs selftest

Media GT has a different base for MOCS register, need to apply
gsi_offset to the mmio address if not using the intel_uncore_r/w
functions for register access.

Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Fei Yang <fei.yang@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230421182535.292670-3-andi.shyti@linux.intel.com