OMAP: DSS2: OMAPFB: swap front and back porches for both hsync and vsync

Framebuffer's left and right margins are relative to the active pixel
area. Front and back porches are relative to the sync area.

Left margin was wrongly assigned to front porch (and right to back),
this patch fixes it.

Signed-off-by:  <tasskjapp@gmail.com>
Reviewed-by:  <Russ.Dill@gmail.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>