drm/syncobj: extract two helpers from drm_syncobj_create

For amdgpu.

drm_syncobj_create is renamed to drm_syncobj_create_as_handle, and new
helpers drm_syncobj_create and drm_syncobj_get_handle are added.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>