The constant N value (0x8000) is used by i915 DP
driver. Define this value in dp helper header file
to use in multiple Display Port drivers. Change
i915 driver accordingly.

Change in v6: Change commit message
Signed-off-by: NChandan Uddaraju <chandanu@codeaurora.org>
Signed-off-by: NVara Reddy <varar@codeaurora.org>
Signed-off-by: NTanmay Shah <tanmay@codeaurora.org>
Reviewed-by: NStephen Boyd <swboyd@chromium.org>
Acked-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NRob Clark <robdclark@chromium.org>