drm/i915/icl: Allocate DSI hosts and imlement host transfer

Allocate DSI host structure for each DSI port available on gen11 and
register them with DSI fwk of DRM. Some of the DSI host operations are
also registered as part of this.

Retrieves DSI pkt (from DSI msg) to be sent over DSI link using DRM DSI
exported functions. A wrapper function is also added as "DSI host
transfer" for sending DSI data/cmd. Add DSI packet payload to command
payload queue using credit based mechanism for *long* packets.

v2 by Jani:
 - indentation
 - Use the new credit available helper
 - Use int for free_credits
 - Add intel_dsi local variable for better code flow
 - Use the new credit available helper
 - Use int for free_credits, i, and j

v3 by Jani:
 - Squash DSI host allocation and transfer patches together

Signed-off-by: Madhav Chauhan <madhav.chauhan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/af4f168ed8737d44687d8b6f21ecaa7e805eb695.1543500286.git.jani.nikula@intel.com