fbdev: imxfb: switch to DEFINE_SIMPLE_DEV_PM_OPS

SIMPLE_DEV_PM_OPS is deprecated, replace it with DEFINE_SIMPLE_DEV_PM_OPS
and use pm_sleep_ptr for setting the driver's pm routines. We can now
remove the __maybe_unused qualifier in the suspend and resume functions.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Helge Deller <deller@gmx.de>