drm: remove allow_fb_modifiers

The allow_fb_modifiers flag is unnecessary since it has been replaced
with fb_modifiers_not_supported flag.

v3:
 - change the order as follows:
   1. add fb_modifiers_not_supported flag
   2. add default modifiers
   3. remove allow_fb_modifiers flag

v5:
 - keep a sanity check in plane init func

Signed-off-by: Tomohito Esaki <etom@igel.co.jp>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220128060836.11216-4-etom@igel.co.jp

drivers/gpu/drm/selftests/test-drm_framebuffer.c

@@ -323,7 +323,6 @@ static struct drm_device mock_drm_device = {
 		.max_width = MAX_WIDTH,
 		.min_height = MIN_HEIGHT,
 		.max_height = MAX_HEIGHT,
-		.allow_fb_modifiers = true,
 		.funcs = &mock_config_funcs,
 	},
 };

include/drm/drm_mode_config.h

@@ -917,22 +917,6 @@ struct drm_mode_config {
 	 */
 	bool async_page_flip;
 
-	/**
-	 * @allow_fb_modifiers:
-	 *
-	 * Whether the driver supports fb modifiers in the ADDFB2.1 ioctl call.
-	 * Note that drivers should not set this directly, it is automatically
-	 * set in drm_universal_plane_init().
-	 *
-	 * IMPORTANT:
-	 *
-	 * If this is set the driver must fill out the full implicit modifier
-	 * information in their &drm_mode_config_funcs.fb_create hook for legacy
-	 * userspace which does not set modifiers. Otherwise the GETFB2 ioctl is
-	 * broken for modifier aware userspace.
-	 */
-	bool allow_fb_modifiers;
-
 	/**
 	 * @fb_modifiers_not_supported:
 	 *