drm/i915/gtt: Reduce a pair of runtime asserts

We can stop asserting using WARN_ON as given sufficient CI coverage, we
can rely on using GEM_BUG_ON() to catch problems before merging.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180614184218.1606-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -2792,7 +2792,7 @@ int i915_gem_init_aliasing_ppgtt(struct drm_i915_private *i915)
 	if (IS_ERR(ppgtt))
 		return PTR_ERR(ppgtt);
 
-	if (WARN_ON(ppgtt->vm.total < ggtt->vm.total)) {
+	if (GEM_WARN_ON(ppgtt->vm.total < ggtt->vm.total)) {
 		err = -ENODEV;
 		goto err_ppgtt;
 	}

drivers/gpu/drm/i915/i915_gem_gtt.h

@@ -464,8 +464,8 @@ static inline u32 i915_pte_count(u64 addr, u64 length, unsigned int pde_shift)
 	const u64 mask = ~((1ULL << pde_shift) - 1);
 	u64 end;
 
-	WARN_ON(length == 0);
-	WARN_ON(offset_in_page(addr|length));
+	GEM_BUG_ON(length == 0);
+	GEM_BUG_ON(offset_in_page(addr | length));
 
 	end = addr + length;