drm/i915: add more debug info for when atomic updates fail v3

I used these additional fields to track down the issue I saw on HSW.

v2: move debug fields into a substruct (Ville)
v3: clean up debug code more (Ville)

References: https://bugs.freedesktop.org/show_bug.cgi?id=91579
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index e9e5cfd..8760814 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -565,8 +565,12 @@
 
 	int scanline_offset;
 
-	unsigned start_vbl_count;
-	ktime_t start_vbl_time;
+	struct {
+		unsigned start_vbl_count;
+		ktime_t start_vbl_time;
+		int min_vbl, max_vbl;
+		int scanline_start;
+	} debug;
 
 	struct intel_crtc_atomic_commit atomic;