drm/i915/selftests: Fixup GuC FW negative test

drm/i915/selftests: Fixup GuC FW negative test

Since:
0d4b78b3d2c0 ("drm/i915/guc: Assert we have the doorbell before setting it up")

We have asserts in GuC doorbell related functions, which is a good thing.
Unfortunately, we were using those to check whether GuC FW is refusing
to allocate invalid doorbell - which makes the test fail.
Well, it would make the test WARN, except we fumbled cleanup ordering
and eat the BUG_ON instead.
Let's keep the asserts and use the internal implementation in the test.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107186
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180712112013.3253-1-chris@chris-wilson.co.uk
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>