drm/i915/selftests: Use igt_random_offset()

Switch igt_vm_isolation() to using igt_random_offset().
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Akeem G Abodunrin <akeem.g.abodunrin@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200312154708.1720-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gem/selftests/i915_gem_context.c

@@ -1809,7 +1809,6 @@ static int igt_vm_isolation(void *arg)
 
 	vm_total = ctx_vm(ctx_a)->total;
 	GEM_BUG_ON(ctx_vm(ctx_b)->total != vm_total);
-	vm_total -= I915_GTT_PAGE_SIZE;
 
 	count = 0;
 	num_engines = 0;
@@ -1828,10 +1827,10 @@ static int igt_vm_isolation(void *arg)
 			u32 value = 0xc5c5c5c5;
 			u64 offset;
 
-			div64_u64_rem(i915_prandom_u64_state(&prng),
-				      vm_total, &offset);
-			offset = round_down(offset, alignof_dword);
-			offset += I915_GTT_PAGE_SIZE;
+			/* Leave enough space at offset 0 for the batch */
+			offset = igt_random_offset(&prng,
+						   I915_GTT_PAGE_SIZE, vm_total,
+						   sizeof(u32), alignof_dword);
 
 			err = write_to_scratch(ctx_a, engine,
 					       offset, 0xdeadbeef);