drm/i915/display: Remove unused POWER_DOMAIN_MASK

That macro became unused with commit 323286c8 ("drm/i915: Move the
power domain->well mappings to intel_display_power_map.c").
Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230815201344.298573-1-gustavo.sousa@intel.com

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

@@ -338,8 +338,6 @@ void intel_display_power_set_target_dc_state(struct drm_i915_private *dev_priv,
 	mutex_unlock(&power_domains->lock);
 }
 
-#define POWER_DOMAIN_MASK (GENMASK_ULL(POWER_DOMAIN_NUM - 1, 0))
-
 static void __async_put_domains_mask(struct i915_power_domains *power_domains,
 				     struct intel_power_domain_mask *mask)
 {