drm/vc4: checking for NULL instead of IS_ERR

vc4_plane_init() returns an ERR_PTR on error, it doesn't return NULL.
This was obviously intended because the next lines call
PTR_ERR(primary_plane) already.

Fixes: c8b75bca ('Eric Anholt <eric@anholt.net>')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Eric Anholt <eric@anholt.net>