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
liboverlay: do not return error if fb num >= 2 doesn't exist (writeback).

Some devices doesn't have writeback support, but needs HDMI (fb1)
This will allow them not to break HDMI support.
This is a cleaner implementation as it won't change handling on
8660 devices having writeback (if those exists).

Change-Id: I6c09759dfa4f2b1734f85012cab4eb4f350820ea
Merge remote-tracking branch 'cm/cm-11.0' into HEAD
Merge branch 'kk44' of https://github.com/VanirAOSP/hardware_qcom_display-caf into HEAD