drm/msm: Refcount framebuffer pins

We were already keeping a refcount of # of prepares (pins), to clear the
iova array.  Use that to avoid unpinning the iova until the last cleanup
(unpin).  This way, when msm_gem_unpin_iova() actually tears down the
mapping, we won't have problems if the fb is being scanned out on
another display (for example).

Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>
Tested-by: Antonino Maniscalco <antomani103@gmail.com>
Reviewed-by: Antonino Maniscalco <antomani103@gmail.com>
Patchwork: https://patchwork.freedesktop.org/patch/661477/

Author: Rob Clark

drivers/gpu/drm/msm/msm_fb.c

@@ -85,7 +85,8 @@ int msm_framebuffer_prepare(struct drm_framebuffer *fb, bool needs_dirtyfb)
 	if (needs_dirtyfb)
 		refcount_inc(&msm_fb->dirtyfb);
 
-	atomic_inc(&msm_fb->prepare_count);
+	if (atomic_inc_return(&msm_fb->prepare_count) > 1)
+		return 0;
 
 	for (i = 0; i < n; i++) {
 		ret = msm_gem_get_and_pin_iova(fb->obj[i], vm, &msm_fb->iova[i]);
@@ -108,11 +109,13 @@ void msm_framebuffer_cleanup(struct drm_framebuffer *fb, bool needed_dirtyfb)
 	if (needed_dirtyfb)
 		refcount_dec(&msm_fb->dirtyfb);
 
+	if (atomic_dec_return(&msm_fb->prepare_count))
+		return;
+
+	memset(msm_fb->iova, 0, sizeof(msm_fb->iova));
+
 	for (i = 0; i < n; i++)
 		msm_gem_unpin_iova(fb->obj[i], vm);
-
-	if (!atomic_dec_return(&msm_fb->prepare_count))
-		memset(msm_fb->iova, 0, sizeof(msm_fb->iova));
 }
 
 uint32_t msm_framebuffer_iova(struct drm_framebuffer *fb, int plane)