drm/i915: Make *_crtc_mode_set work on new_config

This shouldn't change the behavior of those functions, since they are
called after the new_config is made effective and that points to the
current config. In a follow up patch, the mode set sequence will be
changed so this is called before disabling crtcs, and in that case
those functions should work on the staged config.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
[danvet: Flatten if by moving the check into the WARN.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>