OMAPFB: Asynchronous vsync notification

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

Change-Id: I3a40523d35e2eff3510892a4adcd6a4821c883a5
Signed-off-by: Erik Gilling <konkers@android.com>
Signed-off-by: Tony Lofthouse <a0741364@ti.com>
OMAPFB: Select correct VSYNC IRQ based on DSS channel

Select correct VSYNC IRQ for asynchronous vsync notification
based on the DSS channel.
Current VSYNC notification supports only primary LCD panel.
To support VSYNC notification for default HDMI display this
patch adds the VSYNC IRQ selection based on the DSS channel
for default display.

Change-Id: I0594e80d4b4850c278e6bdd3b07c0cb63488be39
Signed-off-by: Muralidhar Dixit <murali.dixit@ti.com>