drm/i915: Add fetch_and_zero() macro

A simple little macro to clear a pointer and return the old value. This
is useful for writing

	value = *ptr;
	if (!value)
		return;

	*ptr = 0;
	...
	free(value);

in a slightly more concise form:

	value = fetch_and_zero(ptr);
	if (!value)
		return;

	...
	free(value);

with the idea that this establishes a pattern that may be extended for
atomic use (using xchg or cmpxchg) i.e. atomic_fetch_and_zero() and
similar to llist.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471254551-25805-10-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_drv.h

@@ -3920,4 +3920,10 @@ bool i915_memcpy_from_wc(void *dst, const void *src, unsigned long len);
 #define ptr_pack_bits(ptr, bits)					\
 	((typeof(ptr))((unsigned long)(ptr) | (bits)))
 
+#define fetch_and_zero(ptr) ({						\
+	typeof(*ptr) __T = *(ptr);					\
+	*(ptr) = (typeof(*ptr))0;					\
+	__T;								\
+})
+
 #endif