drm/i915/selftests: Silence the compiler for impossible errors

It should be impossible for these tests not to run due to an empty
ppgtt, but if it should happen, let's report ENODEV (our typical
internal error for impossible events).

In file included from drivers/gpu/drm/i915/i915_gem.c:5415:
   drivers/gpu/drm/i915/selftests/huge_pages.c: In function 'igt_mock_ppgtt_huge_fill':
>> drivers/gpu/drm/i915/selftests/huge_pages.c:612: error: 'err' may be used uninitialized in this function
   drivers/gpu/drm/i915/selftests/huge_pages.c: In function 'igt_ppgtt_exhaust_huge':
   drivers/gpu/drm/i915/selftests/huge_pages.c:1159: error: 'err' may be used uninitialized in this function

Reported-by:  <kbuild-all@01.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171017103723.6933-1-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.auld@intel.com>