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 <[e-mail]>
Reviewed-by: Javier Martinez Canillas <[e-mail]>
Reviewed-by: Zack Rusin <[e-mail]>
Tested-by: Sui Jingfeng <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/tiny/bochs.c

···
 
 	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

···
 	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

···
 	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:
 	 *
 	 * Special hack for legacy ADDFB to keep nouveau userspace happy. Should