msm: mdss: Add debugfs support for panel command data type

Register debugfs node to read from and write to the panel command
data type. The default data type is DCS_LONG_WRITE 0x39.

Give following command in adb shell to read panel register:
	cat /sys/kernel/debug/mdp/panel_cmd_data_type
To write panel command data type:
	echo "command_data_type" >
		/sys/kernel/debug/mdp/panel_cmd_data_type

Change-Id: I6dbe5bccb3142e93400825eddf7f05180acfc710
Signed-off-by: Yang Xu <yangxu@codeaurora.org>