drm: Make drm_match_cea_mode() return the underlying 2D VIC for 3d modes

When scanning out a stereo mode, the AVI infoframe vic field has to be
the underlyng 2D VIC. Before that commit, we weren't matching the CEA
mode because of the extra stereo flag and then were setting the VIC
field in the AVI infoframe to 0.

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>