drm: exynos: dsi: Use drm panel_bridge API

Replace the manual panel handling code by a drm panel_bridge via
devm_drm_of_get_bridge().

Adding panel_bridge handling,

- Drops drm_connector and related operations as drm_bridge_attach
  creates connector during attachment.

- Drops panel pointer and iterate the bridge, so-that it can operate
  the normal bridge and panel_bridge in constitutive callbacks.

This simplifies the driver and allows all components in the display
pipeline to be treated as bridges.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220303163654.3381470-5-jagan@amarulasolutions.com