drm/crtc_helper: Reset empty plane state in drm_helper_crtc_mode_set_base()

Transitional drivers might access the NULL pointer plane->state in
drm_helper_crtc_mode_set_base(), which causes NULL pointer dereference.
So, let's reset it before handing it over to those drivers.
commit e4f31ad2 ("drm: reset empty state in transitional helpers")
did the same thing for other transitional helpers, but it seems this one
was missed.

Signed-off-by: Liu Ying <gnuiyl@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1459846239-8946-1-git-send-email-gnuiyl@gmail.com