drm/i915: Don't show the blank process name for internal/simulated errors

For a simulated preemption reset, we don't populate the request and so
do not fill in the guilty context name.

[   79.991294] i915 0000:00:02.0: GPU HANG: ecode 9:1:e757fefe, in  [0]

Just don't mention the empty string in the logs!

Fixes: 742379c0 ("drm/i915: Start chopping up the GPU error capture")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200121132107.267709-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gpu_error.c

@@ -1681,7 +1681,7 @@ static const char *error_msg(struct i915_gpu_coredump *error)
 			"GPU HANG: ecode %d:%x:%08x",
 			INTEL_GEN(error->i915), engines,
 			generate_ecode(first));
-	if (first) {
+	if (first && first->context.pid) {
 		/* Just show the first executing process, more is confusing */
 		len += scnprintf(error->error_msg + len,
 				 sizeof(error->error_msg) - len,