drm/exynos: drop exynos_drm_plane_config structure

drm/exynos: drop exynos_drm_plane_config structure

exynos_drm_plane_config must be present for every plane, and most fields
are redundant with exynos_drm_plane:
- pixel_formats, num_pixel_formats are stored in plane.base.format_*,
- type is stored in plane.base.type,
- zpos is always equal to plane.index.
The only non-redundant field capabilities can be moved to exynos_drm_plane.
As consequence of removing the structure some code should be refactored.
The patch should not have functional changes.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>