drm: add constant N value in helper file

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: Chandan Uddaraju <chandanu@codeaurora.org>
Signed-off-by: Vara Reddy <varar@codeaurora.org>
Signed-off-by: Tanmay Shah <tanmay@codeaurora.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>

drivers/gpu/drm/i915/display/intel_display.c

@@ -8114,7 +8114,7 @@ static void compute_m_n(unsigned int m, unsigned int n,
 	 * which the devices expect also in synchronous clock mode.
 	 */
 	if (constant_n)
-		*ret_n = 0x8000;
+		*ret_n = DP_LINK_CONSTANT_N_VALUE;
 	else
 		*ret_n = min_t(unsigned int, roundup_pow_of_two(n), DATA_LINK_N_MAX);
 

include/drm/drm_dp_helper.h

@@ -1134,6 +1134,7 @@
 #define DP_MST_PHYSICAL_PORT_0 0
 #define DP_MST_LOGICAL_PORT_0 8
 
+#define DP_LINK_CONSTANT_N_VALUE 0x8000
 #define DP_LINK_STATUS_SIZE	   6
 bool drm_dp_channel_eq_ok(const u8 link_status[DP_LINK_STATUS_SIZE],
 			  int lane_count);