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

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

BugLink: https://bugs.launchpad.net/bugs/1837517
[ Upstream commit 948dff7cfa1d7653e7828e7b905863bd24ca5c02 ]

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: 7fc7af649ca7 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>