drm: vblank: remove drm_timestamp_monotonic parameter

There is a risk of overflowing vblank timestamps in 2038 or 2106 if
someone sets the drm_timestamp_monotonic module parameter to zero.

I found no indication of anyone ever setting the parameter, or
complaining about the default being wrong, after it was introduced
as a way to handle backwards-compatibility with linux prior to
c61eef72 ("drm: add support for monotonic vblank timestamps"),
so it's probably safer to just remove the parameter completely
and only allowing the default behavior.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>