drm/vc4: Fix race between page flip completion event and clean-up

drm/vc4: Fix race between page flip completion event and clean-up

commit 26fc78f6fef39b9d7a15def5e7e9826ff68303f4 upstream.

There was a small window where a userspace program could submit
a pageflip after receiving a pageflip completion event yet still
receive EBUSY.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Cc: Amit Pundir <amit.pundir@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>