compositor-drm: Comment struct members

Clarify the difference between crtc_id (DRM object) and pipe (index into
drmModeRes->crtcs array, possible_crtcs bitmask).

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Armin Krezović <krezovic.armin@gmail.com>
Differential Revision: https://phabricator.freedesktop.org/D1405

libweston/compositor-drm.c

@@ -155,8 +155,8 @@ struct drm_output {
 	struct weston_output base;
 	drmModeConnector *connector;
 
-	uint32_t crtc_id;
+	uint32_t crtc_id; /* object ID to pass to DRM functions */
-	int pipe;
+	int pipe; /* index of CRTC in resource array / bitmasks */
 	uint32_t connector_id;
 	drmModeCrtcPtr original_crtc;
 	struct drm_edid edid;