drm/dsi: Implement set tear scanline

Provide a small convenience wrapper that transmits
a set_tear_scanline command.

v2:
  * helper function suggested by Thierry
    for set_tear_scanline
  * Also includes small build fixes from Sumit Semwal.

v3: one scanline parameter suggested by jani

v4: passing the payload properly as suggested by jani

Cc: Archit Taneja <archit.taneja@gmail.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Vinay Simha BN <simhavcs@gmail.com>
Reviewed-by: Sumit Semwal <sumit.semwal@linaro.org>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465285532-12676-1-git-send-email-simhavcs@gmail.com