drm/dp: Redo drm_dp_downstream_max_clock() as drm_dp_downstream_max_dotclock()

drm/dp: Redo drm_dp_downstream_max_clock() as drm_dp_downstream_max_dotclock()

We want to differentiate between the DFP dotclock and TMDS clock
limits. Let's convert the current thing to just give us the
dotclock limit.

v2: Use Returns: for kdoc (Lyude)
    Fix up nouveau code too

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200904115354.25336-9-ville.syrjala@linux.intel.com
Reviewed-by: Lyude Paul <lyude@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>