drm/i915/selftests: Use prepare/finish during atomic reset test

We were testing full GPU reset in atomic context without correctly
wrapping it by prepare/finish steps. This could confuse our GuC
reset handling code.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190522193203.23932-4-michal.wajdeczko@intel.com