hwc: Blank and unblank primary on first boot

I made this commit to workaround a strange issue we faced on our Samsung Galaxy S Plus (GT-I9001)
after switching to ION and updated Adreno drivers (Vanilla drivers released on April 2013 by Qualcomm).

After completing the boot and before the first screen off, sometimes the boot splash screen appeared
during app transitions, instead of completely blank screen, for example when starting video playback,
while the first load of the keyboard... The problem happened on CM 10.2 and CM 11.0, not on CM 10.1.

I haven't found another solution at the moment, but probably we're missing something on kernel side (framebuffer driver).

Change-Id: I246858f216d58e2e96a1faf1afac60372f3ddcab