drm/i915: make intel_display_power_domain_str() static

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

Suggested-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240815120002.3472727-1-jani.nikula@intel.com


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