drm/i915: push DDI CRT underrun reporting on enable to encoder

CRT being the only PCH encoder, we can simplify the crtc enable by
pushing the PCH encoder specific parts to CRT encoder.

v2: add separate hsw_enable_crt (Daniel), rebase

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b117fb524fc571837c83292194fc2ad35e588ec9.1507200657.git.jani.nikula@intel.com