drm/i915/dsi: drop unused but set variable data

Prepare for re-enabling -Wunused-but-set-variable.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/a1a167a4ff18b19d10769d83670e414586c16956.1685119007.git.jani.nikula@intel.com