drm/vc4: Add async update support for cursor planes

Now that cursors are implemented as regular planes, all cursor
movements result in atomic updates. As the firmware-kms driver
doesn't support asynchronous updates, these are synchronous, which
limits the update rate to the screen refresh rate. Xorg seems unaware
of this (or at least of the effect of this), because if the mouse is
configured with a higher update rate than the screen then continuous
mouse movement results in an increasing backlog of mouse events -
cue extreme lag.

Add minimal support for asynchronous updates - limited to cursor
planes - to eliminate the lag.

See: https://github.com/raspberrypi/linux/pull/4971
     https://github.com/raspberrypi/linux/issues/4988



Signed-off-by: Phil Elwell <phil@raspberrypi.com>