[media] vsp1: use __maybe_unused for PM handlers

Building without CONFIG_PM results in a harmless warning from
slightly incorrect #ifdef guards:

drivers/media/platform/vsp1/vsp1_drv.c:525:12: error: 'vsp1_pm_runtime_resume' defined but not used [-Werror=unused-function]
drivers/media/platform/vsp1/vsp1_drv.c:516:12: error: 'vsp1_pm_runtime_suspend' defined but not used [-Werror=unused-function]

This removes the existing #ifdef and instead marks all four
PM functions as __maybe_unused.

Fixes: 1e6af546 ("[media] v4l: vsp1: Implement runtime PM support")

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>