video: xilinxfb: Use devm_kzalloc instead of kzalloc

Simplify driver probe and release function.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>