drm/i915/dsi: wait for header and payload credit available

Driver should wait for free header or payload buffer in FIFO.
It would be good to wait a while for HW to release credit before
give it up to write to HW. Without sending initailize command
sets completely. It would caused MIPI display can't light up properly.

Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Cooper Chiou <cooper.chiou@intel.com>
Cc: William Tseng <william.tseng@intel.com>
Signed-off-by: Lee Shawn C <shawn.c.lee@intel.com>
Reviewed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210908115607.9633-2-shawn.c.lee@intel.com