drm/syncobj: Add a CREATE_SIGNALED flag

This requests that the driver create the sync object such that it
already has a signaled dma_fence attached.  Because we don't need
anything in particular (just something signaled), we use a dummy null
fence.  This is useful for Vulkan which has a similar flag that can be
passed to vkCreateFence.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>