drm/i915: Prefer crtc_{h|v}display for pipe src dimensions

Now that we ask to adjust the crtc timings for stereo modes, the correct
pipe_src_w and pipe_src_h can be found in crtc_vdisplay and crtc_hdisplay.

v2: Add comment about why pipe_src_w/h need to be set afert
    set_crtcinfo() (Daniel Vetter)

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>