drm/atomic: Delete an unnecessary check before drm_property_unreference_blob()

The drm_property_unreference_blob() function tests whether its argument
is NULL and then returns immediately.
Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/aa4cd508-38c3-78d7-a9f2-70e3b06a8fb5@users.sourceforge.net