drm/i915: Add new helper to return current attached encoder for connector

For introducing splitted encoder/connector structure, this helper will return
connector's attached encoder when needed.

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>

diff --cc drivers/gpu/drm/i915/intel_display.c
Simple merge

diff --cc drivers/gpu/drm/i915/intel_drv.h
Simple merge