drm/i915: Read out display FIFO size on VLV/CHV

VLV/CHV have similar DSPARB registers as older platforms, just more of
them due to more planes. Add a bit of code to read out the current FIFO
split from the registers. Will be useful later when we improve the WM
calculations.

v2: Add display_mmio_offset to DSPARB

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>