drm/i915: Added port as parameter to the functions which does read/write of DSI Controller

This patch is in preparation of DSI dual link panels. For dual link
panels, few packets needs to be sent to Port A or Port C or both. Based
on the portno from MIPI Sequence Block#53, these sequences needs to be
sent accordingly.

v2: Addressed review comments by Jani
    - port variables named properly

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