Add support for fbdev mmap clients who don't send damage ioctls

Because DisplayLink devices are out on the other end of usb, their
"framebuffer" is just normal system memory. So memory mapped writes
don't automatically trigger anything. So up to this point, standard
fbdev clients who rely on mmap() will get an unchanging screen.

This patch makes udlfb a client of Jaya Kumar's defio framework - which sets
up page fault triggers, and those faults are accumulated and sent to udlfb
on a defferred basis, to process as damage notifications for the framebuffer.

Because this involves more overhead than a notification directly from
the application (e.g. just passing on X DAMAGE extension notifications),
a sysfs attribute is provided by udlfb to control defio support.

/sys/class/graphics/fb*/use_defio - writing a "0" to this file before
calling mmap() causes defio to not be initialized - instead udlfb
will rely on getting damage notifications directly through the damage ioctl.

There are unsolved rendering problems with defio (horizontal dead regions
on framebuffer, that accumulate over time) which still needs a fix.
Signed-off-by: Bernie Thompson <bernie@plugable.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>