drm: Do not set outparam on error during GEM handle allocation

Good practice dictates that we do not leak stale information to our
callers, and should avoid overwriting an outparam on an error path.

Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1451986951-3703-1-git-send-email-chris@chris-wilson.co.uk


Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>