drm: bridge: ti-sn65dsi83: Retrieve the display mode from the state

Instead of storing a copy of the display mode in the sn65dsi83
structure, retrieve it from the atomic state in
sn65dsi83_atomic_enable(). This allows the removal of the .mode_set()
operation, and completes the transition to the atomic API.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210621125518.13715-6-laurent.pinchart@ideasonboard.com