drm/xen: adjust Kconfig

By having selected DRM_XEN, I was assuming I would build the frontend
driver. As it turns out this is a dummy option, and I have really not
been building this (because I had DRM disabled). Make it a promptless
one, moving the "depends on" to the other, real option, and "select"ing
the dummy one.
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Link: https://patchwork.freedesktop.org/patch/msgid/54ae54f9-1ba9-900b-a56f-f48e2c9a82b0@suse.com

drivers/gpu/drm/xen/Kconfig

 # SPDX-License-Identifier: GPL-2.0-only
 config DRM_XEN
-	bool "DRM Support for Xen guest OS"
-	depends on XEN
-	help
-	  Choose this option if you want to enable DRM support
-	  for Xen.
+	bool
 
 config DRM_XEN_FRONTEND
 	tristate "Para-virtualized frontend driver for Xen guest OS"
-	depends on DRM_XEN
-	depends on DRM
+	depends on XEN && DRM
+	select DRM_XEN
 	select DRM_KMS_HELPER
 	select VIDEOMODE_HELPERS
 	select XEN_XENBUS_FRONTEND