hwc: Reset ROI for all the prepare calls.

Partial update ROI needs to be reset at the beginning of every
frame even when we don't have any application layers.

Change-Id: Ib4b604dfa9b589e59150ecc66f48a396f0b37af5
Merge pull request #1 from CyanogenMod/cm-11.0

hwc: Reset ROI for all the prepare calls.