omaplfb_linux: fix build error due omap_dss_device API change

Mainline commit a73fdc64 ("OMAPDSS: rename omap_dss_device's 'output' to 'src'")
renamed OMAP DSS device output member to src.

Change the driver according to the new structure layout to fix
a build error.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>