drm/dsi: Make set_tear_scanline command consistent

Use a consistent name for the function that implements set_tear_scanline
and reword and reformat the kerneldoc slightly.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20160613133327.7630-1-thierry.reding@gmail.com