Sprinkle igt_assert(ptr) after gem_mmap__{cpu,gtt,wc}

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>

tests/gen7_forcewake_mt.c

@@ -191,6 +191,7 @@ igt_simple_main
 		gem_sync(t[0].fd, exec[1].handle);
 
 		p = gem_mmap__gtt(t[0].fd, exec[0].handle, 4096, PROT_READ);
+		igt_assert(p);
 
 		igt_info("[%d]={ %08x %08x }\n", i, p[0], p[1]);
 		igt_assert(p[0] & 2);