drm/amd/display: Add dtbclk access to dcn315

[Why & How]

Currently DCN315 clk manager is missing code to enable/disable dtbclk.
Because of this, "optimized_required" flag is constantly set
and this prevents FreeSync from engaging for certain high bandwidth
display Modes which require DTBCLK.

Reviewed-by: Dmytro Laktyushkin <[e-mail]>
Acked-by: Aurabindo Pillai <[e-mail]>
Signed-off-by: Swapnil Patel <[e-mail]>
Tested-by: Daniel Wheeler <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/dc/clk_mgr/dcn315/dcn315_clk_mgr.c

···
 	 */
 	clk_mgr_base->clks.zstate_support = new_clocks->zstate_support;
 	if (safe_to_lower) {
+		if (clk_mgr_base->clks.dtbclk_en && !new_clocks->dtbclk_en) {
+			dcn315_smu_set_dtbclk(clk_mgr, false);
+			clk_mgr_base->clks.dtbclk_en = new_clocks->dtbclk_en;
+		}
 		/* check that we're not already in lower */
 		if (clk_mgr_base->clks.pwr_state != DCN_PWR_STATE_LOW_POWER) {
 			display_count = dcn315_get_active_display_cnt_wa(dc, context);
···
 			}
 		}
 	} else {
+		if (!clk_mgr_base->clks.dtbclk_en && new_clocks->dtbclk_en) {
+			dcn315_smu_set_dtbclk(clk_mgr, true);
+			clk_mgr_base->clks.dtbclk_en = new_clocks->dtbclk_en;
+		}
 		/* check that we're not already in D0 */
 		if (clk_mgr_base->clks.pwr_state != DCN_PWR_STATE_MISSION_MODE) {
 			union display_idle_optimization_u idle_info = { 0 };