msm: mdss: support enter/exit DCM for calibration

This change supports the enter/exit DCM for calibration tool.
It blocks the unblank operation if target is in DCM (Display
Calibration Mode).

Change-Id: If65449b9863bb1f9ca0346d736b6939261d1394e
Signed-off-by: Arpita Banerjee <cabane@codeaurora.org>

Conflicts:
	drivers/video/msm/mdss/mdss_fb.c