drm/i915: copy&paste drm_crtc_helper_set_mode

Together with the static helper functions drm_crtc_prepare_encoders
and drm_encoder_disable (which will be simplified in the next patch,
but for now are 1:1 copies). Again, no changes beside new names for
these functions.

Also call our new set_mode instead of the crtc helper one now in all
the places we've done so far.

v2: Call the function just intel_set_mode to better differentia it
from intel_crtc_mode_set which really only does the ->mode_set step of
the entire modeset sequence on one crtc. Whereas this function does
the global change.

Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>