drm/i915/dsi: set max return packet size for each dsi port

This seems like the right thing to do. This also gets rid of a call to
intel_dsi_pipe_to_port() which we want to remove eventually.

v2: add braces to fix else logic (Shobhit)

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-By: Shobhit Kumar <shobhit.kumar@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>