drm/i915/dsi: Read/write proper brightness value via MIPI DCS command

Driver has to swap the endian before send brightness level value
to tcon.

v2: Use __be16 instead of u16 to fix sparse warning.
v3: Send one or two bytes brightness value depend on the precision.
v4: get data length of brightness value more easily.

Reported-by: kernel test robot <lkp@intel.com>
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: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210908115607.9633-6-shawn.c.lee@intel.com