drm/msm: Fix dirtyfb refcounting

refcount_t complains about 0->1 transitions, which isn't *quite* what we
wanted.  So use dirtyfb==1 to mean that the fb is not connected to any
output that requires dirtyfb flushing, so that we can keep the underflow
and overflow checking.

Fixes: 9e4dde2 ("drm/msm: Avoid dirtyfb stalls on video mode displays (v2)")
Signed-off-by: Rob Clark <robdclark@chromium.org>
Link: https://lore.kernel.org/r/20220304202146.845566-1-robdclark@gmail.com

drivers/gpu/drm/msm/msm_fb.c

@@ -37,7 +37,7 @@ static int msm_framebuffer_dirtyfb(struct drm_framebuffer *fb,
 	/* If this fb is not used on any display requiring pixel data to be
 	 * flushed, then skip dirtyfb
 	 */
-	if (refcount_read(&msm_fb->dirtyfb) == 0)
+	if (refcount_read(&msm_fb->dirtyfb) == 1)
 		return 0;
 
 	return drm_atomic_helper_dirtyfb(fb, file_priv, flags, color,
@@ -221,6 +221,8 @@ static struct drm_framebuffer *msm_framebuffer_init(struct drm_device *dev,
 		goto fail;
 	}
 
+	refcount_set(&msm_fb->dirtyfb, 1);
+
 	drm_dbg_state(dev, "create: FB ID: %d (%p)", fb->base.id, fb);
 
 	return fb;