drm/panel: panel-dsi-cm: cleanup tear enable

Simplify the code by moving code from _dsicm_enable_te() into
dsicm_power_on().

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-57-tomi.valkeinen@ti.com