msm: mdss: Allow backlight updates in DCM_ENTER case

Modify the check cases in mdss_fb_set_backlight to allow backlight
updates when device is in display calibration mode.

Change-Id: I9fb01f133cc91c2e5e1553e467d5f7ee31086b9d
Signed-off-by: Ping Li <quicpingli@codeaurora.org>