drm: rcar-du: crtc: Support external DSI dot clock

On platforms with an external clock, both the group and crtc must be
handled accordingly to correctly pass through the external clock and
configure the DU to use the external rate.

The CRTC support was missed while adding the DSI support on the r8a779a0
which led to the output clocks being incorrectly determined.

Ensure that when a CRTC is routed through the DSI encoder, the external
clock is used without any further divider being applied.

Fixes: b291fdcf ("drm: rcar-du: Add r8a779a0 device support")
Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>

drivers/gpu/drm/rcar-du/rcar_du_crtc.c

@@ -262,12 +262,13 @@ static void rcar_du_crtc_set_display_timing(struct rcar_du_crtc *rcrtc)
 		rcar_du_group_write(rcrtc->group, DPLLCR, dpllcr);
 
 		escr = ESCR_DCLKSEL_DCLKIN | div;
-	} else if (rcdu->info->lvds_clk_mask & BIT(rcrtc->index)) {
+	} else if (rcdu->info->lvds_clk_mask & BIT(rcrtc->index) ||
+		   rcdu->info->dsi_clk_mask & BIT(rcrtc->index)) {
 		/*
-		 * Use the LVDS PLL output as the dot clock when outputting to
-		 * the LVDS encoder on an SoC that supports this clock routing
-		 * option. We use the clock directly in that case, without any
-		 * additional divider.
+		 * Use the external LVDS or DSI PLL output as the dot clock when
+		 * outputting to the LVDS or DSI encoder on an SoC that supports
+		 * this clock routing option. We use the clock directly in that
+		 * case, without any additional divider.
 		 */
 		escr = ESCR_DCLKSEL_DCLKIN;
 	} else {