drm/vmwgfx: stop using TTM_MEMTYPE_FLAG_MAPPABLE

The driver doesn't expose any not-mapable memory resources.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/378241/