drm/amd: Make sure image is written to trigger VBIOS image update flow

The VBIOS image update flow requires userspace to:
1) Write the image to `psp_vbflash`
2) Read `psp_vbflash`
3) Poll `psp_vbflash_status` to check for completion

If userspace reads `psp_vbflash` before writing an image, it's
possible that it causes problems that can put the dGPU into an invalid
state.

Explicitly check that an image has been written before letting a read
succeed.

Cc: stable@vger.kernel.org
Fixes: 8424f2cc ("drm/amdgpu/psp: Add vbflash sysfs interface support")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -3548,6 +3548,9 @@ static ssize_t amdgpu_psp_vbflash_read(struct file *filp, struct kobject *kobj,
 	void *fw_pri_cpu_addr;
 	int ret;
 
+	if (adev->psp.vbflash_image_size == 0)
+		return -EINVAL;
+
 	dev_info(adev->dev, "VBIOS flash to PSP started");
 
 	ret = amdgpu_bo_create_kernel(adev, adev->psp.vbflash_image_size,