drm/vc4: Move LBM creation out of vc4_plane_mode_set()

We are about to use vc4_plane_mode_set() in the async check path, and
async updates require that LBM size stay the same since they reuse the
LBM from the previous state. So we definitely don't want to allocate a
new LBM region that we know for sure will be free right away.

Move the LBM allocation out of vc4_plane_mode_set() and call the new
function (vc4_plane_update_lbm()) from vc4_plane_atomic_check().

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20181130090254.594-2-boris.brezillon@bootlin.com