Make gem_mmap__{cpu,gtt,wc}() assert on failure

Rename the current gem_mmap__{cpu,gtt,wc}() functions into
__gem_mmap__{cpu,gtt,wc}(), and add back wrappers with the original name
that assert that the pointer is valid. Most callers will expect a valid
pointer and shouldn't have to bother with failures.

To avoid changing anything (yet), sed 's/gem_mmap__/__gem_mmap__/g'
over the entire codebase.

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

tests/gem_largeobject.c

@@ -64,7 +64,7 @@ test_large_object(int fd)
 	igt_assert(ioctl(fd, DRM_IOCTL_I915_GEM_CREATE, &create) == 0);
 
 	/* prefault */
-	ptr = gem_mmap__gtt(fd, create.handle, obj_size, PROT_WRITE | PROT_READ);
+	ptr = __gem_mmap__gtt(fd, create.handle, obj_size, PROT_WRITE | PROT_READ);
 	igt_assert(ptr);
 	*ptr = 0;