drm/dsi: Implement DCS set/get display brightness

Provide a small convenience wrapper that set/get the display brightness.

Cc: John Stultz <john.stultz@linaro.org>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Archit Taneja <archit.taneja@gmail.com>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Vinay Simha BN <simhavcs@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>