drm/i915/selftest: fix an error return path where err is not being set

There is an error condition where err is not being set and an uninitialized
garbage value in err is being returned.  Fix this by assigning err to an
appropriate error return value before taking the error exit path.

Addresses-Coverity: ("Uninitialized scalar value")
Fixes: ed2690a9 ("drm/i915/selftest: Check that GPR are restored across noa_wait")
Signed-off-by: Colin Ian King <colin.king@canonical.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/20200713142551.423649-1-colin.king@canonical.com