drm/atomic: fix null pointer access to mode_fixup callback

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: Inki Dae <inki.dae@samsung.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>