Do the following
 ptr = gem_mmap__{cpu,gtt,wc}()
+igt_assert(ptr);

whenever the code doesn't handle the NULL ptr in any kind of
specific way.

Makes it easier to move the assert into gem_mmap__{cpu,gtt,wc}() itself.

Mostly done with coccinelle, with some manual cleanups:
@@
identifier I;
@@
<... when != igt_assert(I)
     when != igt_require(I)
     when != igt_require_f(I, ...)
     when != I != NULL
     when != I == NULL
(
  I = gem_mmap__gtt(...);
+ igt_assert(I);
|
  I = gem_mmap__cpu(...);
+ igt_assert(I);
|
  I = gem_mmap__wc(...);
+ igt_assert(I);
)
...>

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Stochastically-reviwewed-by: Chris Wilson <chris@chris-wilson.co.uk>