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

drivers/gpu/drm/i915/display/intel_display_power.c

@@ -36,7 +36,7 @@
 	for_each_power_well_reverse(__dev_priv, __power_well)		        \
 		for_each_if(test_bit((__domain), (__power_well)->domains.bits))
 
-const char *
+static const char *
 intel_display_power_domain_str(enum intel_display_power_domain domain)
 {
 	switch (domain) {

drivers/gpu/drm/i915/display/intel_display_power.h

@@ -183,9 +183,6 @@ void intel_display_power_resume(struct drm_i915_private *i915);
 void intel_display_power_set_target_dc_state(struct drm_i915_private *dev_priv,
 					     u32 state);
 
-const char *
-intel_display_power_domain_str(enum intel_display_power_domain domain);
-
 bool intel_display_power_is_enabled(struct drm_i915_private *dev_priv,
 				    enum intel_display_power_domain domain);
 intel_wakeref_t intel_display_power_get(struct drm_i915_private *dev_priv,