drm/i915/selftests: Fix uninitialized variable

There is a potential execution path in which variable err is
returned without being properly initialized previously.

Fix this by initializing variable err to 0.

Addresses-Coverity-ID: 1468362 ("Uninitialized scalar variable")
Fixes: f4ecfbfc ("drm/i915: Check whitelist registers across resets")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
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/20180424131545.GA4053@embeddedor.com