drm/i915/dsi: clean up MIPI DSI pipe vs. port usage

MIPI DSI works on ports A and C, which map to pipes A and B,
respectively. Things are going to get more complicated with the
introduction of dual link DSI support, so clean up the register defines
and code to match reality.

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>