drm/i915/icl: Add macros for MMIO of DSI transcoder registers

This patch adds _MMIO_DSI macros for accessing DSI
transcoder registers.

v2: Use _MMIO_TRANS() (Ville)

Credits-to: Jani N

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Madhav Chauhan <madhav.chauhan@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/3ab94184357d63f2f87b90ef6f5029fb19bef73a.1539613303.git.jani.nikula@intel.com