hwcomposer: Fix compiler warnings

For our prepare and set methods, starting in HWC 1.0 we're
assured that numDisplays is at least 1 and displays[0] is
non-NULL, so we're okay with not explicitly checking that.

We also fix a minor typo while we're in here.

Change-Id: Ia5452a9b3b48e1d6697562a2a9e0ea4d415df5c4
1 file changed