drm/bridge: sii902x: Fix probing race issue

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

drm/bridge: sii902x: Fix probing race issue

A null pointer dereference crash has been observed rarely on TI
platforms using sii9022 bridge:

[ 53.271356] sii902x_get_edid+0x34/0x70 [sii902x]
[ 53.276066] sii902x_bridge_get_edid+0x14/0x20 [sii902x]
[ 53.281381] drm_bridge_get_edid+0x20/0x34 [drm]
[ 53.286305] drm_bridge_connector_get_modes+0x8c/0xcc [drm_kms_helper]
[ 53.292955] drm_helper_probe_single_connector_modes+0x190/0x538 [drm_kms_helper]
[ 53.300510] drm_client_modeset_probe+0x1f0/0xbd4 [drm]
[ 53.305958] __drm_fb_helper_initial_config_and_unlock+0x50/0x510 [drm_kms_helper]
[ 53.313611] drm_fb_helper_initial_config+0x48/0x58 [drm_kms_helper]
[ 53.320039] drm_fbdev_dma_client_hotplug+0x84/0xd4 [drm_dma_helper]
[ 53.326401] drm_client_register+0x5c/0xa0 [drm]
[ 53.331216] drm_fbdev_dma_setup+0xc8/0x13c [drm_dma_helper]
[ 53.336881] tidss_probe+0x128/0x264 [tidss]
[ 53.341174] platform_probe+0x68/0xc4
[ 53.344841] really_probe+0x188/0x3c4
[ 53.348501] __driver_probe_device+0x7c/0x16c
[ 53.352854] driver_probe_device+0x3c/0x10c
[ 53.357033] __device_attach_driver+0xbc/0x158
[ 53.361472] bus_for_each_drv+0x88/0xe8
[ 53.365303] __device_attach+0xa0/0x1b4
[ 53.369135] device_initial_probe+0x14/0x20
[ 53.373314] bus_probe_device+0xb0/0xb4
[ 53.377145] deferred_probe_work_func+0xcc/0x124
[ 53.381757] process_one_work+0x1f0/0x518
[ 53.385770] worker_thread+0x1e8/0x3dc
[ 53.389519] kthread+0x11c/0x120
[ 53.392750] ret_from_fork+0x10/0x20

The issue here is as follows:

- tidss probes, but is deferred as sii902x is still missing.
- sii902x starts probing and enters sii902x_init().
- sii902x calls drm_bridge_add(). Now the sii902x bridge is ready from
DRM's perspective.
- sii902x calls sii902x_audio_codec_init() and
platform_device_register_data()
- The registration of the audio platform device causes probing of the
deferred devices.
- tidss probes, which eventually causes sii902x_bridge_get_edid() to be
called.
- sii902x_bridge_get_edid() tries to use the i2c to read the edid.
However, the sii902x driver has not set up the i2c part yet, leading
to the crash.

Fix this by moving the drm_bridge_add() to the end of the
sii902x_init(), which is also at the very end of sii902x_probe().