drm/i915: Extract intel_encoder_has_connectors()

We have an open coded loop which tries to see if the encoder has any
connectors linked to it. Let's extract that to a helper similar to
intel_crtc_has_encoders().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1455737325-14777-1-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>