drm/vkms: set preferred depth to 24

Otherwise DRM_CAP_DUMB_PREFERRED_DEPTH is zero.
Signed-off-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/o2SmEP0M6h287Fs5OeL3HtQzTr8uV0PyOhFict_qdxeS7vFvV1Zr8qzRUJh-YnCoJHo13nJgAqqQByzEzTZTqk9R2ExsOszYLj68hQLqBIc=@emersion.fr

drivers/gpu/drm/vkms/vkms_drv.c

@@ -95,6 +95,7 @@ static int vkms_modeset_init(struct vkms_device *vkmsdev)
 	dev->mode_config.min_height = YRES_MIN;
 	dev->mode_config.max_width = XRES_MAX;
 	dev->mode_config.max_height = YRES_MAX;
+	dev->mode_config.preferred_depth = 24;
 
 	return vkms_output_init(vkmsdev);
 }