drm/i915: Add missing intel_mchbar_regs.h

The new header from the previous commit didn't get added after a
conflict resolution...let's add it now.

Fixes: e30e6c7b

 ("drm/i915: Move MCHBAR registers to their own header")
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/20220215061342.2055952-2-matthew.d.roper@intel.com