drm: Extract add_hdmi_mode() out of do_hdmi_vsdb_modes()

So we respect a nice design of having similar functions at the same
level, in this case:

do_hdmi_vsdb_modes()
  - add_hdmi_mandatory_stereo_modes()
  - add_hdmi_mode()

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>