drm/exynos: remove exynos_drm_create_enc_conn()

This functions was just hiding the encoder and connector creation in
a way that was less clean than if we get rid of it. For example,
exynos_encoder ops had .create_connector() defined only because we were
handing off the encoder and connector creation to
exynos_drm_create_enc_conn(). Without this function we can directly call
the create_connector function internally in the code, without the need of
any vtable access.

It also does some refactoring in the code like creating a bind function
for dpi devices.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>