udlfb: Add module option to do without shadow framebuffer

By default, udlfb allocates a 2nd buffer to shadow what's across
the bus on the USB device.  It can operate without this shadow,
but then it cannot tell which pixels have changed, and must send all.

Saves host memory, but worsens the USB 2.0 bus bottleneck.

This option allows users in very low memory situations (e.g.
bifferboard) to optionally turn off this shadow framebuffer.

Signed-off-by: Stuart Hopkins <stuart@linux-depot.com>
Signed-off-by: Bernie Thompson <bernie@plugable.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>