msm: mdss: enable backlight gpio after commit is done

Backlight gpio was enabled during panel power on. So backlight
was turned on before commit was sent to the panel. Ensure
backlight gpio is enabled only after the backlight level is set.

Change-Id: I30a0a0cfc1c07761ae2b2bea8424d79e7e22ec42
Signed-off-by: Ashish Garg <ashigarg@codeaurora.org>