drm/i915: Skip allocating shadow batch for 0-length batches

Since

commit 17cabf57
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Wed Jan 14 11:20:57 2015 +0000

    drm/i915: Trim the command parser allocations

we may then try to allocate a zero-sized object and attempt to extract
its pages. Understandably this fails.

Note that the real offender seems to be

commit b9ffd80e
Author: Brad Volkin <bradley.d.volkin@intel.com>
Date:   Thu Dec 11 12:13:10 2014 -0800

    drm/i915: Use batch length instead of object size in command parser

Testcase: igt/gem_exec_nop #ivb,byt,hsw
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[cherry picked from commit 743e78c1
from drm-intel-next because 4.0 seems to be affected by this too,
despite that the obvious culprit is definitely not in 4.0. Whatever,
if fixes a bug.
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>