drm/i915: Fix a NULL vs IS_ERR() bug

The mmap_offset_attach() function returns error pointers, it doesn't
return NULL.

Fixes: eaee1c08 ("drm/i915: Add a function to mmap framebuffer obj")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ZH7tHLRZ9oBjedjN@moroto

drivers/gpu/drm/i915/gem/i915_gem_mman.c

@@ -1085,8 +1085,8 @@ int i915_gem_fb_mmap(struct drm_i915_gem_object *obj, struct vm_area_struct *vma
 		/* handle stolen and smem objects */
 		mmap_type = i915_ggtt_has_aperture(ggtt) ? I915_MMAP_TYPE_GTT : I915_MMAP_TYPE_WC;
 		mmo = mmap_offset_attach(obj, mmap_type, NULL);
-		if (!mmo)
-			return -ENODEV;
+		if (IS_ERR(mmo))
+			return PTR_ERR(mmo);
 	}
 
 	/*