drm/i915/reg: split out icl_dsi_regs.h

The ICL DSI registers have fairly isolated usage. Split the register
macros to a separate file.

Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220217224023.3994777-4-jani.nikula@intel.com