drm/etnaviv: Remove #ifdef guards for PM related functions

Use the RUNTIME_PM_OPS() and pm_ptr() macros to handle the
.runtime_suspend/.runtime_resume callbacks.

These macros allow the suspend and resume functions to be automatically
dropped by the compiler when CONFIG_PM is disabled, without having
to use #ifdef guards.

This has the advantage of always compiling these functions in,
independently of any Kconfig option. Thanks to that, bugs and other
regressions are subsequently easier to catch.

Some #ifdef CONFIG_PM guards were protecting simple statements, and were
also converted to "if (IS_ENABLED(CONFIG_PM))".

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>