fbdev: auo_k190x: avoid unused function warnings

The auo_k190x framebuffer driver encloses the power-management
functions in #ifdef CONFIG_PM, but the auok190x_suspend/resume
functions are only really used when CONFIG_PM_SLEEP is also
set, as a frequent gcc warning shows:

drivers/video/fbdev/auo_k190x.c:859:12: warning: 'auok190x_suspend' defined but not used
drivers/video/fbdev/auo_k190x.c:899:12: warning: 'auok190x_resume' defined but not used

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>