drm/fbdev-generic: Remove unused prefer_shadow_fbdev flag

Remove the flag prefer_shadow_fbdev from struct drm_mode_config.
Drivers set this flag to enable shadow buffering in the generic
fbdev emulation. Such shadow buffering is now mandatory, so the
flag is unused.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Zack Rusin <zackr@vmware.com>
Tested-by: Sui Jingfeng <suijingfeng@loongson.cn>
Link: https://patchwork.freedesktop.org/patch/msgid/20230320150751.20399-3-tzimmermann@suse.de

drivers/gpu/drm/tiny/bochs.c

@@ -545,7 +545,6 @@ static int bochs_kms_init(struct bochs_device *bochs)
 
 	bochs->dev->mode_config.preferred_depth = 24;
 	bochs->dev->mode_config.prefer_shadow = 0;
-	bochs->dev->mode_config.prefer_shadow_fbdev = 1;
 	bochs->dev->mode_config.quirk_addfb_prefer_host_byte_order = true;
 
 	bochs->dev->mode_config.funcs = &bochs_mode_funcs;

drivers/gpu/drm/vmwgfx/vmwgfx_kms.c

@@ -2112,7 +2112,6 @@ int vmw_kms_init(struct vmw_private *dev_priv)
 	dev->mode_config.max_width = dev_priv->texture_max_width;
 	dev->mode_config.max_height = dev_priv->texture_max_height;
 	dev->mode_config.preferred_depth = dev_priv->assume_16bpp ? 16 : 32;
-	dev->mode_config.prefer_shadow_fbdev = !dev_priv->has_mob;
 
 	drm_mode_create_suggested_offset_properties(dev);
 	vmw_kms_create_hotplug_mode_update_property(dev_priv);

include/drm/drm_mode_config.h

@@ -890,13 +890,6 @@ struct drm_mode_config {
 	/* dumb ioctl parameters */
 	uint32_t preferred_depth, prefer_shadow;
 
-	/**
-	 * @prefer_shadow_fbdev:
-	 *
-	 * Hint to framebuffer emulation to prefer shadow-fb rendering.
-	 */
-	bool prefer_shadow_fbdev;
-
 	/**
 	 * @quirk_addfb_prefer_xbgr_30bpp:
 	 *