drm/i915: Ask the DRM core do make stereo timings adjustements

When scanning out big stereo buffers that are actually bigger that their
natural 2D counterpart, we need to blow up the crtc timings as well.

Not that this is only done for frame packing as this is the only stereo
mode currently exposed needing this kind of ajdustements.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>