drm/msm: Add missing stub definition

DRM_MSM fails to build with DRM_MSM_DP=n; add the missing stub.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Fixes: 8ede2ecc ("drm/msm/dp: Add DP compliance tests on
Signed-off-by: Rob Clark <robdclark@chromium.org>