UPSTREAM: drm/mm: avoid possible null pointer dereference

UPSTREAM: drm/mm: avoid possible null pointer dereference

Do not dereference node before the check if node is NULL.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1463602639-4861-1-git-send-email-xypron.glpk@gmx.de
(cherry picked from commit b80d3942329dcd43580adc3f7c0a098515b5feec)

Change-Id: I672ce10214e1268a043e6a11268ff400024cc883
Signed-off-by: Mark Yao <mark.yao@rock-chips.com>