drm/exynos: move connector creation to attach callback

The current implementation assumes that the only possible peripheral
device for DSIM is a panel. Using an output bridge child device
should also be possible.

If an output bridge is available, don't create a new connector.
Instead, call drm_bridge_attach() and set encoder's bridge to NULL
in order to avoid an out bridge from being visible by the framework, as
the DSI bus needs control on enabling its child output bridge.

Such sequence is required by Toshiba TC358764 bridge, which is a DSI
peripheral bridge device.

changed in v5:
- detach bridge in mipi_dsi detach callback

Signed-off-by: Maciej Purski <m.purski@samsung.com>
[ a.hajda@samsung.com: v5 ]
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Manually merged due to merge conflict.
Signed-off-by: Inki Dae <inki.dae@samsung.com>