drm/i915/selftests: avoid using uninitialized context

There is an error path in igt_ppgtt_alloc(), which leads
to ww object being passed down to i915_gem_ww_ctx_fini() without
initialization. Correct that by only putting ppgtt->vm and
returning early.

Fixes: 480ae795 ("drm/i915/selftests: Prepare gtt tests for obj->mm.lock removal")
Signed-off-by: Krzysztof Karas <krzysztof.karas@intel.com>
Reviewed-by: Mikolaj Wasiak <mikolaj.wasiak@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/iuaonpjc3rywmvhna6umjlvzilocn2uqsrxfxfob24e2taocbi@lkaivvfp4777