drm/msm: Fix NULL deref in adreno_load_gpu

The msm/kms driver should work even if there is no GPU device specified
in DT. Currently, we get a NULL dereference crash in adreno_load_gpu
since the driver assumes that priv->gpu_pdev is non-NULL.

Perform an additional check on priv->gpu_pdev before trying to retrieve
the msm_gpu pointer from it.

v2: Incorporate Jordan's comments:
- Simplify the check to share the same error message.
- Use dev_err_once() to avoid an error message every time we open the
  drm device fd.

Fixes: eec874ce (drm/msm/adreno: load gpu at probe/bind time)
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Acked-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>

drivers/gpu/drm/msm/adreno/adreno_device.c

@@ -130,11 +130,14 @@ struct msm_gpu *adreno_load_gpu(struct drm_device *dev)
 {
 	struct msm_drm_private *priv = dev->dev_private;
 	struct platform_device *pdev = priv->gpu_pdev;
-	struct msm_gpu *gpu = platform_get_drvdata(priv->gpu_pdev);
+	struct msm_gpu *gpu = NULL;
 	int ret;
 
+	if (pdev)
+		gpu = platform_get_drvdata(pdev);
+
 	if (!gpu) {
-		dev_err(dev->dev, "no adreno device\n");
+		dev_err_once(dev->dev, "no GPU device was found\n");
 		return NULL;
 	}