drm/i915: Report resv_obj allocation failure

Since commit 64d6c500 ("drm/i915: Generalise GPU activity
tracking"), we have been prepared for i915_vma_move_to_active() to fail.
We can take advantage of this to report the failure for allocating the
shared-fence slot in the reservation_object.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190730205805.3733-1-chris@chris-wilson.co.uk