drm/i915: Track visible planes in a bitmask

In a lot of place we wish to know which planes on the crtc are actually
visible, or how many of them there are. Let's start tracking that in a
bitmask in the crtc state.

We already track enabled planes (ie. ones with an fb and crtc specified by
the user) but that's not quite the same thing as enabled planes may
still end up being invisible due to clipping and whatnot.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170302171508.1666-2-ville.syrjala@linux.intel.com