drm: panfrost: Remove #ifdef guards for PM related functions

Use the EXPORT_GPL_RUNTIME_DEV_PM_OPS() and pm_ptr() macros to handle
the PM callbacks.

These macros allow the PM 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.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Steven Price <steven.price@arm.com>
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221129191942.138244-3-paul@crapouillou.net