[media] s5p-csis: Fix compilation with PM_SLEEP disabled

Fix following compilation error when CONFIG_PM_SLEEP is disabled:

  CC      drivers/media/video/s5p-fimc/mipi-csis.o
drivers/media/video/s5p-fimc/mipi-csis.c: In function ‘s5pcsis_remove’:
drivers/media/video/s5p-fimc/mipi-csis.c:956: error: implicit declaration of function ‘s5pcsis_suspend’

Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>