drm/dp: Add drm_dp_alternate_scrambler_reset_cap() helper

Add a helper to check if the sink supports the eDP alternate scrambler
reset value of 0xfffe.

Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191021143437.1477719-6-thierry.reding@gmail.com