drm/i915: push DDI FDI link training on enable to CRT encoder

Reduce encoder specific checks from CRTC code.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6903136894ffcf47f7511900f3c8d53f760ef6b1.1507200657.git.jani.nikula@intel.com