drm/i915/bdw: Add Broadwell display FIFO limits

Broadwell has bigger display FIFOs than Haswell. Otherwise the
two are very similar.

v2: Fix FBC WM_LP shift for BDW

v3: Rebase on top of the big Haswell wm rework.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com> (v2)
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>