compositor-drm: Fix disabling cursor plane

commit a7cba1d4 changed the way
the cursor plane is setup.  Previously it was pre-emptively set
disabled for the next frame, and that would be changed at next
frame time if the cursor plane was to be used.  It was changed
to be disabled at plane assignment time.

We disable the use of planes entirely by setting disable_planes to
a non-zero value, which bypasses all calls to assign_planes - so
if the plane was set-up in the previous frame it will retain its
state post-disable.

This leads to desktop zoom leaving the cursor plane in place when
it sets disable_planes.

This patch clears any stale cursor plane state from the redraw
handler if disable_planes is set so drm_output_set_cursor()
will do the right thing.

Reviewed-by: Daniel Stone <daniels@collabora.com>
Reported-by: Emmanuel Gil Peyrot <emmanuel.peyrot@collabora.com>