drm/vc4: Don't check plane state more than once

We are about to use vc4_plane_mode_set() in the async check path, but
async check can decide that async update is not possible and force the
driver to fallback to a sync update.

All the checks that have been done on the plane state during async check
stay valid, and checking it again is not necessary. Add a ->checked
field to vc4_plane_state, and use it to track the status of the state
(checked or not).

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20181130090254.594-3-boris.brezillon@bootlin.com