HWC: Minor fix in 4k2kYUVsplit feature

Increment fbZ when 4k2kyuvindex matches with fbZ,
otherwise we see momentary flicker as right half of
4k2k YUV layer and fb layer have same z-order.

Change-Id: Ia900e102b6d4670b9bfdfd4e52ef360d1d813cab
1 file changed