drm/i915: Add output_types bitmask into the crtc state

Rather than looping through encoders to see which encoder types
are being driven by the pipe, add an output_types bitmask into
the crtc state and populate it prior to compute_config and during
state readout.

v2: Determine output_types before .compute_config() hooks are called

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/1466621833-5054-4-git-send-email-ville.syrjala@linux.intel.com