drm/i915/dsi: add separate init timer mask definition for ICL DSI

Having a separate definition will be useful for splitting VLV and ICL
register files.

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/20220217224023.3994777-2-jani.nikula@intel.com