drm/i915: Remove crtc_mode_set() hook

There's no users left after the conversion to calculate clocks before
disabling crtcs during mode set.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>