This patch fixes null pointer access incurred when
encoder driver didn't set its own mode_fixup callback.

mode_fixup callback shoudn't be called if the callback
of drm_encoder_helper_funcs is NULL.

Changelog v2:
- change it to else if
Signed-off-by: NInki Dae <inki.dae@samsung.com>
Reviewed-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>