drm/vc4: update cursors asynchronously through atomic

Add support for async updates of cursors by using the new atomic
interface for that. Basically what this commit does is do what
vc4_update_plane() did but through atomic.

v7: Place the drm_atomic_set_fb_for_plane() call after the new
    FB has been applied to the HW to avoid possible use-after-free
    issues
v6: add missing drm_atomic_set_fb_for_plane() in
    vc4_plane_atomic_async_update() (Boris Brezillon)
v5: add missing call to vc4_plane_atomic_check() (Eric Anholt)
v4: add drm_atomic_helper_async() commit (Eric Anholt)
v3: move size checks back to drivers (Ville Syrjälä)
v2: move fb setting to core and use new state (Eric Anholt)

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180330085445.31726-1-boris.brezillon@bootlin.com