drm/i915: Use atomic helpers for computing changed flags

Replace the drivers own logic for computing mode_changed, active_changed
and planes_changed flags with the check_modeset() atomic helper. Since
that function needs to compare the crtc's new mode with the current,
this patch also moves the set up of crtc_state->mode earlier in the call
chain.

Note that for the call to check_plane() to work properly, we need to
check new plane state against new crtc state. But since we still use the
plane update helper, which doesn't have a full atomic state, we need to
hack around that in intel_plane_atomic_check().

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