drm/i915: Bail out early if WA batch is not available for given Gen

To initialize WA batch, at the moment we first allocate batch and then check
whether we have any WA to be initialized for the given Gen; if we don't have
any WA then we WARN the user, destroy the batch and return but this is causing
another WARN in cleanup code complaining about sleeping in atomic context.
Till we understand this better and to keep things simpler, bail out early
if we don't have WA.

Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Arun Siluvery <arun.siluvery@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>