drm/i915: Promote ptrdiff() to i915_utils.h

Generic helpers should be placed in i915_utils.h.
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210603051630.2635-9-matthew.brost@intel.com

drivers/gpu/drm/i915/i915_utils.h

@@ -201,6 +201,11 @@ __check_struct_size(size_t base, size_t arr, size_t count, size_t *size)
 	__T;								\
 })
 
+static __always_inline ptrdiff_t ptrdiff(const void *a, const void *b)
+{
+	return a - b;
+}
+
 /*
  * container_of_user: Extract the superclass from a pointer to a member.
  *

drivers/gpu/drm/i915/i915_vma.h

@@ -146,11 +146,6 @@ static inline void i915_vma_put(struct i915_vma *vma)
 	i915_gem_object_put(vma->obj);
 }
 
-static __always_inline ptrdiff_t ptrdiff(const void *a, const void *b)
-{
-	return a - b;
-}
-
 static inline long
 i915_vma_compare(struct i915_vma *vma,
 		 struct i915_address_space *vm,