drm/i915/dsi: call dpi_send_cmd() for each dsi port at a higher level

Instead of having the for each dsi port loop within dpi_send_cmd(), add
a port parameter to the function and call it for each port instead.

This is a rewrite of

commit 4510cd77
Author: Gaurav K Singh <gaurav.k.singh@intel.com>
Date:   Thu Dec 4 10:58:51 2014 +0530

    drm/i915: Dual link needs Shutdown and Turn on packet for both ports

to add more flexibility in using dpi_send_cmd() for just one port as
necessary. No functional changes.

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>