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.