drm/omap: dsi: move TE GPIO handling into core

In preparation for removing custom DSS calls from the DSI
panel driver, this moves support for external tearing event
GPIOs into the DSI host driver. This way tearing events are
always handled in the core resulting in simplification of
the panel drivers.

The TE GPIO acquisition follows works in the same way as the
exynos DSI implementation.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-25-tomi.valkeinen@ti.com