drm/i915: passing drm connector param for load detection

In load detection, connector's encoder assignment must be kept
consistent for proper mode setting, and this makes connector as
explicit parameter for load detect function to not require single
data structure to hold both encoder and connector reference, ease
the transition for splitted encoder/connector model.

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