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