drm: Merge CONFIG_DRM_VM into CONFIG_DRM_LEGACY

CONFIG_DRM_VM gets selected by CONFIG_DRM_LEGACY, but nothing else. So
remove it and build drm_vm.o as part of CONFIG_DRM_LEGACY.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210112081035.6882-5-tzimmermann@suse.de