drm/dp: Drop open-coded drm_dp_is_branch() in drm_dp_read_downstream_info()

Noticed this while fixing another issue in drm_dp_read_downstream_info(),
the open coded DP_DOWNSTREAMPORT_PRESENT check here just duplicates what we
already do in drm_dp_is_branch(), so just get rid of it.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210430223428.10514-2-lyude@redhat.com