rpisense-fb: Flush any deferred updates on release

Since [1], writing to the Sense HAT framebuffer and immediately closing
the file descriptor is likely to have no effect. This is because the
framebuffer deferred IO framework is now careful to cancel pending
updates.

To retain the old behaviour, add an fb_release method that flushes the
updates instead.

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")