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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>