drm/i915: Silence allocation failure during sg_trim()

As trimming the sg table is merely an optimisation that gracefully fails
if we cannot allocate a new table, we do not need to report the failure
either.

Fixes: 0c40ce13 ("drm/i915: Trim the object sg table")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161223145804.6605-4-chris@chris-wilson.co.uk