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

In the Linux kernel, the following vulnerability has been resolved:

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.