The driver doesn't support UMS any more, so set DRIVER_MODESET by default,
remove the legacy s/r callbacks, and rename the s/r functions to make it more clear
they're only in use by switcheroo now.

Also remove an obsolete comment about atomic. Normal updates are supported only
async updates aren't yet.

v2: Don't unconditionally set DRIVER_ATOMIC, we're not yet there.
Signed-off-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>