cocoa: Use swapInterval from QSurfaceFormat

xcb and windows already do this. The default value is 1 so the
behavior on cocoa does not change when setSwapInterval() is not
called.

Task-number: QTBUG-31939
Change-Id: Id9d9204c9324f1cc187c6aee4286e7da42583759
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>