drm/i915: rename intel_modes.c to intel_connector.c

The common denominator here seems to be connector more than
modes. Prepare for moving more connector functions to the same place. 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/20181010075205.7713-1-jani.nikula@intel.com