drm/panel: Enable DSI transactions on the RPi panel.

It turns out that I had just mistaken what type of write the register
writes were supposed to be, using DCS instead of generic long writes.

Switching to transactions instead of using the atmel as a bridge also
seems to resolve the sparkling pixels problem I've had.
Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: 2f733d61 ("drm/panel: Add support for the Raspberry Pi 7" Touchscreen.")
Link: https://patchwork.freedesktop.org/patch/msgid/20171031193258.17373-2-eric@anholt.netReviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
Acked-by: Thierry Reding <treding@nvidia.com>

drivers/gpu/drm/panel/panel-raspberrypi-touchscreen.c

@@ -238,12 +238,6 @@ static void rpi_touchscreen_i2c_write(struct rpi_touchscreen *ts,
 
 static int rpi_touchscreen_write(struct rpi_touchscreen *ts, u16 reg, u32 val)
 {
-#if 0
-	/* The firmware uses LP DSI transactions like this to bring up
-	 * the hardware, which should be faster than using I2C to then
-	 * pass to the Toshiba.  However, I was unable to get it to
-	 * work.
-	 */
 	u8 msg[] = {
 		reg,
 		reg >> 8,
@@ -253,13 +247,7 @@ static int rpi_touchscreen_write(struct rpi_touchscreen *ts, u16 reg, u32 val)
 		val >> 24,
 	};
 
-	mipi_dsi_dcs_write_buffer(ts->dsi, msg, sizeof(msg));
-#else
-	rpi_touchscreen_i2c_write(ts, REG_WR_ADDRH, reg >> 8);
-	rpi_touchscreen_i2c_write(ts, REG_WR_ADDRL, reg);
-	rpi_touchscreen_i2c_write(ts, REG_WRITEH, val >> 8);
-	rpi_touchscreen_i2c_write(ts, REG_WRITEL, val);
-#endif
+	mipi_dsi_generic_write(ts->dsi, msg, sizeof(msg));
 
 	return 0;
 }