drm/i915/selftests: Check that GPR are cleared for new contexts

We want the general purpose registers to be clear in all new contexts so
that we can be confident that no information is leaked from one to the
next.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191014090757.32111-7-chris@chris-wilson.co.uk