gma500: frame buffer locking

If we are the console then a printk can hit us with a spin lock held (and
in fact the kernel will do its best to take the console printing lock).

In that case we cannot politely sleep when synching after an accelerated op
but must behave obnoxiously to be sure of getting the bits out.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>