drm/dp: Add drm_dp_fast_training_cap() helper

Add a helper that checks for the fast training capability given the DPCD
receiver capabilities blob.

Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191021143437.1477719-4-thierry.reding@gmail.com