drm/i915/mtl: Introduce FBC B

MTL introduces a second FBC engine. The two FBC engines can
operate entirely independently, FBC A serving pipe A and
FBC B serving pipe B.

The one place where things might go a bit wrong is the CFB
allocation from stolen. We might have to consider some change
to the allocation strategy to have a better chance of both
engines being able to allocate its CFB. Maybe FBC A should
allocate bottom up and FBC B top down, or something? For the
moment the allocation strategy is DRM_MM_INSERT_BEST for both.

Cc: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220817122624.213889-1-jani.nikula@intel.com