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

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

v2: add hsw_disable_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/1ecbbb946f7b3782ee3d97f0ec2fe4758c349311.1507200657.git.jani.nikula@intel.com