drm/i915: Avoid NULL ctx->obj dereference in debugfs/i915_context_info

In commit 691e6415
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Wed Apr 9 09:07:36 2014 +0100

    drm/i915: Always use kref tracking for all contexts.

we populated fake contexts on all platforms. These were identical to the
full hardware context tracking structs, except for the ctx->obj used to
store the hardware state. However, there remained one place where we
assumed that if a context existed, it would have an object associated
with it.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=77717


Testcase: igt/drv_suspend/debugfs-reader
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>