drm/mipi: use dcs write for mipi_dsi_dcs_set_tear_scanline

The helper uses the MIPI_DCS_SET_TEAR_SCANLINE, although it's currently
using the generic write. This does not look right.

Perhaps some platforms don't distinguish between the two writers?

Cc: Robert Chiras <robert.chiras@nxp.com>
Cc: Vinay Simha BN <simhavcs@gmail.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Thierry Reding <treding@nvidia.com>
Fixes: e8395081 ("drm/dsi: Implement set tear scanline")
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200505160329.2976059-3-emil.l.velikov@gmail.com