gma500: Do sane FB cleanup

If we get a user frame buffer destroyed which is being displayed then clean
up the mess nicely. We can now run a slightly modified modetest including setting
modes, and handling crashes.

Modetest still blows up but this is because libdrm 2.4.25 is busted.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>