drm/i915: move encoder->mode_set calls to crtc_mode_set

Makes more sense to group the entire mode_set stage into one function.
Noticed while discussiing the rather confusing set of function names
with Paulo Zanoni. Unfortunately I don't have an idea to make the
function names lesss confusion.

v2: Use for_each_encoder_on_crtc as suggested by Chris Wilson.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>