hwc/overlay: Validate cursor position in hwc_setPositionAsync

- Validate new cursor position in hwc_setPositionAsync before
using it to avoid unnecessary IOCTL failure.

- To free the cursor, MSMFB_CURSOR IOCTL should be called without
operation FB_CUR_SETIMAGE, so set it to 0.

Change-Id: Iac044832a7bbf30b141eff97f8d57cd78e8b6d0e
2 files changed