drm/i915/debugfs: add crtc i915_pipe debugfs file

The pipe may differ from crtc index if pipes are fused off. For testing
purposes, IGT needs to know the pipe.

There's already a I915_GET_PIPE_FROM_CRTC_ID IOCTL for this. However,
the upcoming Xe driver won't have that IOCTL, and going forward, we'll
want a unified interface for testing i915 and Xe, as they share the
display code. Thus add the debugfs for i915 display.

v2: User letters for pipe names (Ville)

Cc: Bhanuprakash Modem <bhanuprakash.modem@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Bhanuprakash Modem <bhanuprakash.modem@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230320124429.786985-2-jani.nikula@intel.com

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

@@ -1383,6 +1383,17 @@ static int i915_current_bpc_show(struct seq_file *m, void *data)
 }
 DEFINE_SHOW_ATTRIBUTE(i915_current_bpc);
 
+/* Pipe may differ from crtc index if pipes are fused off */
+static int intel_crtc_pipe_show(struct seq_file *m, void *unused)
+{
+	struct intel_crtc *crtc = m->private;
+
+	seq_printf(m, "%c\n", pipe_name(crtc->pipe));
+
+	return 0;
+}
+DEFINE_SHOW_ATTRIBUTE(intel_crtc_pipe);
+
 /**
  * intel_connector_debugfs_add - add i915 specific connector debugfs files
  * @connector: pointer to a registered drm_connector
@@ -1453,4 +1464,6 @@ void intel_crtc_debugfs_add(struct intel_crtc *crtc)
 
 	debugfs_create_file("i915_current_bpc", 0444, root, crtc,
 			    &i915_current_bpc_fops);
+	debugfs_create_file("i915_pipe", 0444, root, crtc,
+			    &intel_crtc_pipe_fops);
 }