drm/msm/sde: Make DRM_MSM_DP depend on USB_PD

Make sure that DRM_MSM_DP depends on USB_PD so it can't be
selected unless USB_PD support is also available and add a
stub for msm_edp_modeset_init() so it can be successfully
compiled if DRM_MSM_EDP is not selected.

Fixes: 1eae18f0 ("drm/msm: Add snapshot of SDE, DSI, DP and supporting files")
Change-Id: Ic0dedbad503ab2f7fa4a73e09d419c5247e3f13e
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>