drm/msm: deal with exhausted vmap space better

Some, but not all, callers of obj->vmap() would check if return
IS_ERR().  So let's actually return an error if vmap() fails.  And fixup
the call-sites that were not handling this properly.

Signed-off-by: Rob Clark <robdclark@gmail.com>

drivers/gpu/drm/msm/msm_gem.c

@@ -398,6 +398,8 @@ void *msm_gem_vaddr_locked(struct drm_gem_object *obj)
 			return ERR_CAST(pages);
 		msm_obj->vaddr = vmap(pages, obj->size >> PAGE_SHIFT,
 				VM_MAP, pgprot_writecombine(PAGE_KERNEL));
+		if (msm_obj->vaddr == NULL)
+			return ERR_PTR(-ENOMEM);
 	}
 	return msm_obj->vaddr;
 }