viu_of_probe() ignores fails in i2c_get_adapter(),
tries to unlock uninitialized mutex on error path.

The patch streamlining the error handling in viu_of_probe().

Found by Linux Driver Verification project (linuxtesting.org).
Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>