drm/panthor: Fix the FW reset logic

In the post_reset function, if the fast reset didn't succeed, we
are not clearing the fast_reset flag, which prevents firmware
sections from being reloaded. While at it, use panthor_fw_stop()
instead of manually writing DISABLE to the MCU_CONTROL register.

Fixes: 2718d91816 ("drm/panthor: Add the FW logical block")
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240430113727.493155-1-boris.brezillon@collabora.com

drivers/gpu/drm/panthor/panthor_fw.c

@@ -1083,10 +1083,11 @@ int panthor_fw_post_reset(struct panthor_device *ptdev)
 		if (!ret)
 			goto out;
 
-		/* Force a disable, so we get a fresh boot on the next
-		 * panthor_fw_start() call.
+		/* Forcibly reset the MCU and force a slow reset, so we get a
+		 * fresh boot on the next panthor_fw_start() call.
 		 */
-		gpu_write(ptdev, MCU_CONTROL, MCU_CONTROL_DISABLE);
+		panthor_fw_stop(ptdev);
+		ptdev->fw->fast_reset = false;
 		drm_err(&ptdev->base, "FW fast reset failed, trying a slow reset");
 	}