drm/i915: Polish possible_clones setup

Replace the hand rolled stuff with drm_encoder_mask() when populating
possible_clones, and rename the function to
intel_encoder_possible_clones() to make it clear what it's used for.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191002162505.30716-1-ville.syrjala@linux.intel.com


Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>