drm: msm: sde: reduce black screen duration from null commit

User may send null commit without any plane attached to kernel.
In kernel, these null commits will clear mixer blendstage for
all pipes, and power on screen with black background color.
However, when bootloader splash is on, this operation will
override the splash, which brings long black screen duration
between splash and user UI. So this patch is to fix this.

Change-Id: I3a34ab2ad421f40bd315eb2874fea5dc33d3ccfb
Signed-off-by: Guchun Chen <guchunc@codeaurora.org>