drm/dsi: Flag for non-continuous clock behavior

As per section 5.6.1 of the DSI specification, all DSI transmitters must
support continuous clock behavior on the clock lane, while non-continuous
mode support is only optional. Add a flag that allows devices to indicate
that they support non-continuous clock mode so host drivers can adapt
their behavior accordingly.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>