drm/i915: add pipe_config->pixel_multiplier

Used by SDVO (and hopefully, eventually HDMI, if we ever get around
to fixing up the low dotclock CEA modes ...).

This required adding a new encoder->mode_set callback to be able to
pass around the intel_crtc_config.

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