drm/i915: Reworkd DP DFP clock handling

Move the downstream facing port dotclock check into a new function
(intel_dp_mode_valid_downstream()) so that we have a nice future
place where we can collect other related checks.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200904115354.25336-10-ville.syrjala@linux.intel.com


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