drm/i915/dmc: fix i915_reg_t usage

i915_reg_t is supposed to be a somewhat opaque data type, not to be
looked inside.
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b813d0cf1b800386742eb3f919b0ba8d5c182a2a.1647870374.git.jani.nikula@intel.com

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

@@ -816,7 +816,7 @@ static int intel_dmc_debugfs_status_show(struct seq_file *m, void *unused)
 	struct drm_i915_private *i915 = m->private;
 	intel_wakeref_t wakeref;
 	struct intel_dmc *dmc;
-	i915_reg_t dc5_reg, dc6_reg = {};
+	i915_reg_t dc5_reg, dc6_reg = INVALID_MMIO_REG;
 
 	if (!HAS_DMC(i915))
 		return -ENODEV;
@@ -868,7 +868,7 @@ static int intel_dmc_debugfs_status_show(struct seq_file *m, void *unused)
 	}
 
 	seq_printf(m, "DC3 -> DC5 count: %d\n", intel_de_read(i915, dc5_reg));
-	if (dc6_reg.reg)
+	if (i915_mmio_reg_valid(dc6_reg))
 		seq_printf(m, "DC5 -> DC6 count: %d\n",
 			   intel_de_read(i915, dc6_reg));