fbdev: Don't cancel deferred work if pagelist empty

Since [1], the fbdev deferred IO framework is careful to cancel
pending updates on close to prevent dirty pages being accessed after
they may have been reused. However, this is not necessary in the case
that the pagelist is empty, and drivers that don't make use of the
pagelist may have wanted updates cancelled for no good reason.

Avoid penalising fbdev drivers that don't make use of the pagelist by
making the cancelling of deferred IO on close conditional on there
being a non-empty pagelist.

See: https://github.com/raspberrypi/linux/issues/5398



Signed-off-by: Phil Elwell <phil@raspberrypi.com>

[1] 3efc61d9 ("fbdev: Fix invalid page access after closing deferred I/O devices")