drm/i915/crt: clean up encoder hook assignment

Only assign the hooks once instead of overwriting for DDI.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/796b631d018fdd5aa2a3608c6a9b9fa196a50f76.1507200657.git.jani.nikula@intel.com