drm/i915/dsi: use min_t() to make code cleaner

Use min_t() in order to make code cleaner.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Changcheng Deng <deng.changcheng@zte.com.cn>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220308091655.2078825-1-deng.changcheng@zte.com.cn