drm: Add dirty ioctl and property

This commit adds a ioctl and property to allow userspace
to notify the kernel that a framebuffer has changed. Instead
of snooping the command stream this allows finer grained
tracking of which areas have changed.

The primary user for this functionality is virtual hardware
like the vmware svga device, but also Xen hardware likes to
be notify. There is also real hardware like DisplayLink and
DisplayPort that might take advantage of this ioctl.

Signed-off-by: Jakob Bornecrantz <jakob@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>