drm/bridge: megachips: Fix a null pointer dereference bug

When removing the module we will get the following warning:

[   31.911505] i2c-core: driver [stdp2690-ge-b850v3-fw] unregistered
[   31.912484] general protection fault, probably for non-canonical address 0xdffffc0000000001: 0000 [#1] PREEMPT SMP KASAN PTI
[   31.913338] KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f]
[   31.915280] RIP: 0010:drm_bridge_remove+0x97/0x130
[   31.921825] Call Trace:
[   31.922533]  stdp4028_ge_b850v3_fw_remove+0x34/0x60 [megachips_stdpxxxx_ge_b850v3_fw]
[   31.923139]  i2c_device_remove+0x181/0x1f0

The two bridges (stdp2690, stdp4028) do not probe at the same time, so
the driver does not call ge_b850v3_resgiter() when probing, causing the
driver to try to remove the object that has not been initialized.

Fix this by checking whether both the bridges are probed.

Fixes: 11632d4a ("drm/bridge: megachips: Ensure both bridges are probed before registration")
Signed-off-by: Zheyu Ma <zheyuma97@gmail.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220830073450.1897020-1-zheyuma97@gmail.com

drivers/gpu/drm/bridge/megachips-stdpxxxx-ge-b850v3-fw.c

@@ -296,7 +296,9 @@ static void ge_b850v3_lvds_remove(void)
 	 * This check is to avoid both the drivers
 	 * removing the bridge in their remove() function
 	 */
-	if (!ge_b850v3_lvds_ptr)
+	if (!ge_b850v3_lvds_ptr ||
+	    !ge_b850v3_lvds_ptr->stdp2690_i2c ||
+		!ge_b850v3_lvds_ptr->stdp4028_i2c)
 		goto out;
 
 	drm_bridge_remove(&ge_b850v3_lvds_ptr->bridge);