drm: Release reference from blob lookup after replacing property

drm_property_lookup_blob() returns a reference to the returned blob, and
drm_atomic_replace_property_blob() takes a references to the blob it
stores, so afterwards we are left owning a reference to the new_blob that
we never release, and thus leak memory every time we update a property
such as during drm_atomic_helper_legacy_gamma_set().

v2: update credentials, drm_property_unreference_blob() is NULL safe and
NULL is passed consistently to it throughout drm_atomic.c so do so here.

Reported-by: Felix Monninger <felix.monninger@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98420
Signed-off-by: Felix Monninger <felix.monninger@gmail.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org
Fixes: 5488dc16

 ("drm: introduce pipe color correction properties")
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161025212808.3908-1-chris@chris-wilson.co.uk