drm/bridge: ti-sn65dsi83: Make enable GPIO optional

The enable signal may not be controllable by the kernel. Make it
optional.
This is a similar to commit bbda1704 ("drm/bridge: ti-sn65dsi86: Make
enable GPIO optional")

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211213133626.2498056-3-alexander.stein@ew.tq-group.com