drm/radeon: Populate crtc_clock in radeon_atom_get_tv_timings()

crtc_clock is now supposed to be the actual pixel clock corresponding to
the other crtc_ timing values. Populate crtc_clock appropriately in
radeon_atom_get_tv_timings().

This was the only obvious place where we frob with the crtc_ timigns
directly instead of calling drm_mode_set_crtcinfo() which would also
update crtc_clock.

Reviewed-by:  <mario.kleiner.de@gmail.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>