drm/i915: rename and move intel_get_pipe_from_connector()

Rename intel_get_pipe_from_connector() to intel_connector_get_pipe() and
move it near its connector function friends in intel_connector.c. 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/20181016145044.3924-1-jani.nikula@intel.com