drm/amd/display: remove oem i2c adapter on finish

Fixes a bug where unbinding of the GPU would leave the oem i2c adapter
registered resulting in a null pointer dereference when applications try
to access the invalid device.

Fixes: 3d5470c9 ("drm/amd/display/dm: add support for OEM i2c bus")
Cc: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Geoffrey McRae <geoffrey.mcrae@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>