drm/i915: Switch to fake context on older gens

drm/i915: Switch to fake context on older gens

We used to have per file descriptor hang stats for the
i915_get_reset_stats_ioctl() and for default context banning.

commit 0eea67eb26000657079b7fc41079097942339452
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Fri Dec 6 14:11:19 2013 -0800

    drm/i915: Create a per file_priv default context

made having separate hangstats in file_private redundant
as i915_hw_context already contained hangstats. So

commit c482972a086e03e6a6d27e4f7af2d868bf659648
Author: Ben Widawsky <benjamin.widawsky@intel.com>
Date:   Fri Dec 6 14:11:20 2013 -0800

    drm/i915: Piggy back hangstats off of contexts

consolidated the hangstats and enabled further improvements.

commit 44e2c0705a19e09d7b0f30a591f92e473e5ef89e
Author: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Date:   Thu Jan 30 16:01:15 2014 +0200

    drm/i915: Use i915_hw_context to set reset stats

tried to reap full benefits of consolidation but fell short
as we never 'switch' to the fake private context on gens
that don't have hw_contexts, so request->ctx remained NULL
on those.

Fix this by 'switching' to fake context so that when
request is submitted to ring, proper context gets assigned
to it.

Testcase: igt/drv_hangman
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76055
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>