drm/i915: Repeat the GEM_BUG_ON message in the ftrace log

As the ftrace log is overflowing the pstore capture, we lose the last
gasps from dmesg which includes the GEM_BUG_ON function:line and condition
that failed. Vital information for tracking down the bug, so append it to
the frace log as well.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180227211816.5546-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.h

@@ -29,7 +29,10 @@
 
 #ifdef CONFIG_DRM_I915_DEBUG_GEM
 #define GEM_BUG_ON(condition) do { if (unlikely((condition))) {	\
-		printk(KERN_ERR "GEM_BUG_ON(%s)\n", __stringify(condition)); \
+		pr_err("%s:%d GEM_BUG_ON(%s)\n", \
+		       __func__, __LINE__, __stringify(condition)); \
+		GEM_TRACE("%s:%d GEM_BUG_ON(%s)\n", \
+			  __func__, __LINE__, __stringify(condition)); \
 		BUG(); \
 		} \
 	} while(0)