fbdev: sm712fb: avoid unused function warnings

The sm712fb framebuffer driver encloses the power-management
functions in #ifdef CONFIG_PM, but the smtcfb_pci_suspend/resume
functions are only really used when CONFIG_PM_SLEEP is also
set, as a frequent gcc warning shows:

fbdev/sm712fb.c:1549:12: warning: 'smtcfb_pci_suspend' defined but not used
fbdev/sm712fb.c:1572:12: warning: 'smtcfb_pci_resume' defined but not used

The driver also avoids using the SIMPLE_DEV_PM_OPS macro when
CONFIG_PM is unset, which is redundant.

This changes the driver to remove the #ifdef and instead mark
the functions as __maybe_unused, which is a nicer anyway, as it
provides build testing for all the code in all configurations
and is harder to get wrong.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>