drm/i915: kill a no longer necessary BUG_ON

i915_gem_object_move_to_active can handle zero seqno for us now.
And not emitting a request is not fatal here - we'll try to emit
a new one if we have to wait for some rendering to complete.

In case this assumption ever gets accidentally broken, there's already
a BUG_ON to catch it in i915_do_wait_request.

So just silently ignore ENOMEM here instead of screwing up the whole
drm.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>