drm/i915: Define MCH registers relative to MCHBAR_MIRROR_BASE

A few of our MCH registers are defined with absolute register offsets.
For consistency, let's switch their definitions to be relative offsets
from MCHBAR_MIRROR_BASE.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220215061342.2055952-1-matthew.d.roper@intel.com