drm/exynos: balance framebuffer refcount

exynos_drm_crtc_mode_set assigns primary framebuffer to plane without
taking reference. Then during framebuffer removal it is dereferenced twice,
causing oops. The patch fixes it.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>