udlfb: allow reallocating the framebuffer

This patch changes udlfb so that it may reallocate the framebuffer when
setting higher-resolution mode. If we boot the system without monitor
attached, udlfb creates a framebuffer with the size 800x600. This patch
makes it possible to select higher videomode with the fbset command when
a monitor is attached.

Note that there is no reliable way to prevent the system from touching the
old framebuffer, so we must not free it. We add it to the list
dlfb->deferred_free and free it when the driver is unloaded.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
[b.zolnierkie: sparse fixes]
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>