drm: use dma_fence_unwrap_merge() in drm_syncobj

The unwrap merge function is now intended for this use case.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220518135844.3338-6-christian.koenig@amd.com