media: ipu3-imgu: Remove dead code for NULL check

Since ipu3_css_buf_dequeue() never returns NULL, remove the
dead code to fix static checker warning:

drivers/staging/media/ipu3/ipu3.c:493 imgu_isr_threaded()
warn: 'b' is an error pointer or valid

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
[Bug report: https://lore.kernel.org/linux-media/20190104122856.GA1169@kadam/

]
Signed-off-by: Yong Zhi <yong.zhi@intel.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>