drm: add definitions for DP Audio/Video compliance tests

This change adds definitions needed for DP audio compliance testing.
It also adds missing definition for DP video compliance.

Changes in V2:
-- Delete cover letter for this patch.
-- Move the description from cover letter into patch commit message.
-- Remove DPU from subject prefix

Reviewed-by: Sean Paul <sean@poorly.run>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch> # for merging through -msm.
Signed-off-by: Chandan Uddaraju <chandanu@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>