drm/i915/selftest: Fix error handling in igt_vma_remapped_gtt()

An inner scope version of err shadows the variable in the outer scope,
and err doesn't get set after a failure, fix these.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210325214808.2071517-3-imre.deak@intel.com