drm/i915/dsi: add ports to intel_dsi to describe the ports being driven

Later on this can include multiple ports (e.g. (1 << PORT_A) | (1 <<
PORT_C)) to describe dual link DSI.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Gaurav K Singh <gaurav.k.singh@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>