drm: Remove clock_index from struct drm_display_mode

This field was only accessed by the nouveau driver, but never set. So
concluded we can rid of this one.

Acked-by: Ben Skeggs <bskeggs@redhat.com>
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>