drm/msm/dsi-staging: update mdp transfer time preference

With this change, mdp transfer time updated to userspace
will be the preferred dtsi entry, when both dsi clock
and mdp transfer time nodes are set.

Change-Id: I37cd55e3d6f3f0f78f4ca4bddf921f6cf743c1b9
Signed-off-by: Vara Reddy <varar@codeaurora.org>