drm/i915/selftest: Refactor reset locking

drm/i915/selftest: Refactor reset locking

Extract the common barrier against rogue hangchecks from disrupting our
direct testing of resets, and in the process expand the lock to include
the per-engine reset shortcuts.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170721123238.16428-17-chris@chris-wilson.co.uk
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>