drm/i915: move intel connector specific functions to intel_connector.c

Now that we have intel_connector.c, move the connector specific
functions from intel_display.c there. Fix a few checkpatch complaints
while at it. No functional changes.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181010075205.7713-2-jani.nikula@intel.com