drm/dp: Add drm_dp_downstream_{min,max}_tmds_clock()

Add helpers to get the TMDS clock limits for HDMI/DVI downstream
facing ports.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200904115354.25336-11-ville.syrjala@linux.intel.com


Reviewed-by: Lyude Paul <lyude@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>