drm/i915/bdw: add Broadwell sprite/plane/cursor checks

Just make Broadwell follow the same code paths as Haswell here,
instead of running code for the even-older platforms.

v2: Shuffle around Ben's vma prep work.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>