drm/i915/selftests: Always initialise err

drm/i915/selftests: Always initialise err

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.uk
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>