msm: mdss: add debugfs support for LCM debug

Register debugfs nodes to read from and write to the LCM registers.
Give following command in adb shell to read panel register:
	echo address feedback_count > /d/mdp/panel_off
	cat /d/mdp/panel_reg
To write panel register:
	echo address value > /d/mdp/panel_reg

Change-Id: I9ef9fb57dc1b7dea42c7e701c60018f7946ee75b
Signed-off-by: Ray Zhang <rayz@codeaurora.org>