drm/i915: Bail out early on gen6_signal if no semaphores

If we dont have semaphores enabled, we allocate 4
dwords for signalling. But end up emitting more regardless.

Fix this by bailing out early if semaphores are not enabled.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=78274
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=78283


Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>