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: NChris 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.ukReviewed-by: NMatthew Auld <matthew.auld@intel.com>

drivers/gpu/drm/i915/selftests/huge_pages.c

@@ -609,7 +609,7 @@ static int igt_mock_ppgtt_huge_fill(void *arg)
 	bool single = false;
 	LIST_HEAD(objects);
 	IGT_TIMEOUT(end_time);
-	int err;
+	int err = -ENODEV;
 
 	for_each_prime_number_from(page_num, 1, max_pages) {
 		struct drm_i915_gem_object *obj;
@@ -1157,7 +1157,7 @@ static int igt_ppgtt_exhaust_huge(void *arg)
 	unsigned int size_mask;
 	unsigned int page_mask;
 	int n, i;
-	int err;
+	int err = -ENODEV;
 
 	/*
 	 * Sanity check creating objects with a varying mix of page sizes --