drm/i915: make __intel_display_power_is_enabled() static

The function isn't used outside of intel_display_power.c. Make it
static.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240813151216.2573845-1-jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>