drm/i915/selftests: Do not leak vm_area_struct on early return

This structure may be leaked on early failure paths, so include
vm_munmap() call in them to avoid that.

Suggested-by: Chris Wilson <chris.p.wilson@linux.intel.com>
Signed-off-by: Krzysztof Karas <krzysztof.karas@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://lore.kernel.org/r/d8277e3af09c388fa5dfbf5c9bb3985ae0b191a2.1753859971.git.krzysztof.karas@intel.com