video:uvesafb: notice user when we failed to save hardware state

uvesafb_open may failed to save hardware state when kmalloc failed
in uvesafb_vbe_state_save, we should check this and notice user.

Signed-off-by: Wang YanQing <udknight@gmail.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>