drm/i915: Check max number of lanes when registering DDI ports

DDI A and E share some of the lanes, so check that we have enough
lanes for the purpose we need before registering the encoders.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449597590-6971-3-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>