video: fbdev: pxafb: fix out of memory error path

As seen by Julia, the initial allocation memory is not checked anymore
after commit "video: fbdev: pxafb: initial devicetree conversion".
Introduce back the removed test.

Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>