drm/i915/selftests: Add a simple exerciser for suspend/hibernate

Although we cannot do a full system-level test of suspend/hibernate from
deep with the kernel selftests, we can exercise the GEM subsystem in
isolation and simulate the external effects (such as losing stolen
contents and trashing the register state).

v2: Don't forget to hold rpm
v3: Suspend the GTT mappings, and more rpm!

References: https://bugs.freedesktop.org/show_bug.cgi?id=96526
References: 5ab57c70

 ("drm/i915: Flush logical context image out to memory upon suspend")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jakub Bartmiński <jakub.bartminski@intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Jakub Bartmiński <jakub.bartminski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180830134806.21939-1-chris@chris-wilson.co.uk