media: staging/intel-ipu3: mark PM function as __maybe_unused

The imgu_rpm_dummy_cb() looks like an API misuse that is explained
in the comment above it. Aside from that, it also causes a warning
when power management support is disabled:

drivers/staging/media/ipu3/ipu3.c:794:12: error: 'imgu_rpm_dummy_cb' defined but not used [-Werror=unused-function]

The warning is at least easy to fix by marking the function as
__maybe_unused.

Fixes: 7fc7af64 ("media: staging/intel-ipu3: Add imgu top level pci device driver")

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>