drm/vc4: Fix atomic_async_check to call the right mode_set function

vc4_plane_atomic_async_check was always calling vc4_plane_mode_set
to validate and generate the dlist for the check. If async_check
decided it had to fall back to a sync commit, then this GEN4/5
dlist could get used on GEN6.

Call either vc4_plane_mode_set or vc6_plane_mode_set as appropriate.

Fixes: 1ab1fbbb

 ("drm/vc4: hvs: Support BCM2712 HVS")
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>