drm/gem: Clear mapping addresses for unused framebuffer planes

Set the returned mapping address to NULL if a framebuffer plane does
not have a BO associated with it. Likewise, ignore mappings of NULL
during framebuffer unmap operations. Allows users of the functions to
perform unmap operations of certain BOs by themselfes.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210730183511.20080-4-tzimmermann@suse.de

drivers/gpu/drm/drm_gem_framebuffer_helper.c

@@ -335,8 +335,10 @@ int drm_gem_fb_vmap(struct drm_framebuffer *fb,
 
 	for (i = 0; i < DRM_FORMAT_MAX_PLANES; ++i) {
 		obj = drm_gem_fb_get_obj(fb, i);
-		if (!obj)
+		if (!obj) {
+			dma_buf_map_clear(&map[i]);
 			continue;
+		}
 		ret = drm_gem_vmap(obj, &map[i]);
 		if (ret)
 			goto err_drm_gem_vunmap;
@@ -376,6 +378,8 @@ void drm_gem_fb_vunmap(struct drm_framebuffer *fb,
 		obj = drm_gem_fb_get_obj(fb, i);
 		if (!obj)
 			continue;
+		if (dma_buf_map_is_null(&map[i]))
+			continue;
 		drm_gem_vunmap(obj, &map[i]);
 	}
 }