drm: eliminate old_fb from drm_crtc_helper_set_config

Old framebuffer is stored in save_set.fb and it is
the same value that is later stored in old_fb.
This makes old_fb redundant so we can replace
it with save_set.fb.

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>