drm/vc4: Make sure vc4_bo_{inc,dec}_usecnt() calls are balanced

Commit b9f19259 ("drm/vc4: Add the DRM_IOCTL_VC4_GEM_MADVISE ioctl")
introduced a mechanism to mark some BOs as purgeable to allow the driver
to drop them under memory pressure. In order to implement this feature
we had to add a mechanism to mark BOs as currently used by a piece of
hardware which materialized through the ->usecnt counter.

Plane code is supposed to increment usecnt when it attaches a BO to a
plane and decrement it when it's done with this BO, which was done in
the ->prepare_fb() and ->cleanup_fb() hooks. The problem is, async page
flip logic does not go through the regular atomic update path, and
->prepare_fb() and ->cleanup_fb() are not called in this case.

Fix that by manually calling vc4_bo_{inc,dec}_usecnt() in the
async-page-flip path.

Note that all this should go away as soon as we get generic async page
flip support in the core, in the meantime, this fix should do the
trick.

Fixes: b9f19259

 ("drm/vc4: Add the DRM_IOCTL_VC4_GEM_MADVISE ioctl")
Reported-by: Peter Robinson <pbrobinson@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180430133232.32457-1-boris.brezillon@bootlin.com
Link: https://patchwork.freedesktop.org/patch/msgid/20180430133232.32457-1-boris.brezillon@bootlin.com