drm/i915: Move MCHBAR registers to their own header

Registers that exist within the MCH BAR and are mirrored into the GPU's
MMIO space are a good candidate to separate out into their own header.

For reference, the mirror of the MCH BAR starts at the following
locations in the graphics MMIO space (the end of the MCHBAR range
differs slightly on each platform):

 * Pre-gen6:           0x10000
 * Gen6-Gen11 + RKL:  0x140000

v2:
 - Create separate patch to swtich a few register definitions to be
   relative to the MCHBAR mirror base.
 - Drop upper bound of MCHBAR mirror from commit message; there are too
   many different combinations between various platforms to list out,
   and the documentation is spotty for the older pre-gen6 platforms
   anyway.

Bspec: 134, 51771
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/20220215061342.2055952-2-matthew.d.roper@intel.com