smatch does not track initialised values as well as gcc, and this
triggers many warnings by smatch not presented by gcc. Silence smatch by
initialising the error values to -ENODEV, which we use to denote
internal errors. (If we see a selftest fail with a silent -ENODEV, we
know smatch was right!)

v2: smatch was right about igt_create_vma(), it may unlikely fail on the
first object allocation which we want to be loud about.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171114223346.25958-1-chris@chris-wilson.co.ukReviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>