OMAPFB: Asynchronous vsync notification

Enabled and disabled through new OMAPFB_ENABLEVSYNC ioctl.  Events delivered
as switch events.

Change-Id: Ie473d96732c5bf2259bac6205d0295d698e6e48b
Signed-off-by: Erik Gilling <konkers@android.com>
Signed-off-by: Tony Lofthouse <a0741364@ti.com>