video: xilinxfb: Move xilinxfb_platform_data directly to the driver

No reason to have separate file in header in include/linux folder
if this is purely driver specific structure.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>