drm/i915: Create a intel_encoder_find_connector helper function.

drm/i915: Create a intel_encoder_find_connector helper function.

This makes the code in intel_sanitize_encoder slightly more readable.
This was meant to be addressed in fd6bbda9c7a4, but I missed that
review comment.

Fixes: fd6bbda9c7a4 ("drm/i915: Pass crtc_state and connector_state to encoder functions")
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471961888-10771-1-git-send-email-maarten.lankhorst@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[mlankhorst: Fix unused variable reported by kbuild.]