drm/i915/selftest: Fix an error code in live_noa_gpr()

The error code needs to be set on this path.  It currently returns
success.

Fixes: ed2690a9 ("drm/i915/selftest: Check that GPR are restored across noa_wait")
Reported-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200714143652.GA337376@mwanda