drm/amdgpu: clarify why we evict vram twice on suspend

Update the comment to explain why we do this.
Reviewed-by: NChristian König <christian.koenig@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_device.c

@@ -1982,7 +1982,10 @@ int amdgpu_device_suspend(struct drm_device *dev, bool suspend, bool fbcon)
 
 	r = amdgpu_suspend(adev);
 
-	/* evict remaining vram memory */
+	/* evict remaining vram memory
+	 * This second call to evict vram is to evict the gart page table
+	 * using the CPU.
+	 */
 	amdgpu_bo_evict_vram(adev);
 
 	pci_save_state(dev->pdev);