drm/i915: Add crtc->plane_ids_mask

Add a mask of which planes are available for each pipe. This doesn't
quite work for old platforms with dynamic plane<->pipe assignment, but
as we don't support that sort of stuff (yet) we can get away with it.

The main use I have for this is the for_each_plane_id_on_crtc() macro
for iterating over all possible planes on the crtc. I suppose we could
not add the mask, and instead iterate by comparing intel_plane->pipe
but then we'd need a local intel_plane variable which is just
unnecessary clutter in some cases. But I'm not hung up on this, so if
people prefer the other option I could be convinced to use it.

v2: Use BIT() in the iterator macro too (Paulo)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479830524-7882-3-git-send-email-ville.syrjala@linux.intel.com