drm/exynos: remove struct exynos_drm_display

This struct was just representing encoder information, it was a member of
struct exynos_drm_encoder, so any code trying to access encoder data would
have to go through the encoder struct, get the display struct and then get
the data it want.

During this patchset we also realized that the only data
exynos_drm_encoder needs to store is the drm_encoder parent and the
exynos_drm_encoder_ops.

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