drm/udl: Remove flags field from struct udl_gem_object

The flags field in struct udl_gem controls mapping parameters: cached
access for local buffers, write-combined access for imported buffers.

We can drop the field and distinguish both cases by testing whether
struct drm_gem_object.import_attach is NULL.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107094307.19870-2-tzimmermann@suse.de