drm/i915: fix i915_interrupt_info on BDW

Currently, if the machine is runtime suspended an you read the file,
you will get an "Unclaimed register" error message.

Testcase: igt/pm_rpm/debugfs-read
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_debugfs.c

@@ -703,6 +703,12 @@ static int i915_interrupt_info(struct seq_file *m, void *data)
 		}
 
 		for_each_pipe(pipe) {
+			if (!intel_display_power_enabled(dev_priv,
+						POWER_DOMAIN_PIPE(pipe))) {
+				seq_printf(m, "Pipe %c power disabled\n",
+					   pipe_name(pipe));
+				continue;
+			}
 			seq_printf(m, "Pipe %c IMR:\t%08x\n",
 				   pipe_name(pipe),
 				   I915_READ(GEN8_DE_PIPE_IMR(pipe)));