drm: introduce a capability flag for syncobj timeline support

Unfortunately userspace users of this API cannot be publicly disclosed
yet.

This commit effectively disables timeline syncobj ioctls for all
drivers. Each driver wishing to support this feature will need to
expose DRIVER_SYNCOBJ_TIMELINE.

v2: Add uAPI capability check (Christian)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com> (v1)
Cc: Dave Airlie <airlied@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Christian König <christian.koenig@amd.com>
Cc: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190416125750.31370-1-lionel.g.landwerlin@intel.com