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 <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Reviewed-by: Laurent Pinchart <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

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

···
 		.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

···
 	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:
 	 *
 	 * When this flag is set, the DRM device will not expose modifier