drm/i915/gt: Remove #ifdef guards for PM related functions

Instead of defining two versions of intel_sysfs_rc6_init(), one for each
value of CONFIG_PM, add a check on !IS_ENABLED(CONFIG_PM) early in the
function. This will allow the compiler to automatically drop the dead
code 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: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221129191942.138244-13-paul@crapouillou.net