drm/i915: HSW+ FBC is tied to pipe A

So add code to consider this case.

v2: Reorder the series, so drop the possible_framebuffer_bits chunk.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com> (v1)
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>