drm/bridge: tc358764: restore connector support

This patch restores DRM connector registration in the TC358764 bridge
driver and restores usage of the old drm_panel_* API, thus allows dynamic
panel registration. This fixes panel operation on Exynos5250-based
Arndale board.

This is equivalent to the revert of the following commits:
1644127f "drm/bridge: tc358764: add drm_panel_bridge support"
385ca38d "drm/bridge: tc358764: drop drm_connector_(un)register"
and removal of the calls to drm_panel_attach()/drm_panel_detach(), which
were no-ops and has been removed in meanwhile.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200930114042.5806-1-m.szyprowski@samsung.com