drm/rockchip: return ERR_PTR instead of NULL

rockchip_drm_framebuffer_init is only used in one case, in
rockchip_drm_fbdev.c, where its return value is tested using IS_ERR.  To
enable propagating the reason for the error, change the definition so that
it returns an ERR_PTR value.

Problem found with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>