drm/exynos: g2d: Convert to driver component API

Exynos G2D driver is the last client of the custom Exynos 'sub-driver'
framework. In the current state it doesn't really resolve any of the
issues it has been designed for, as Exynos DRM is already built only
as a single kernel module. Remove the custom 'sub-driver' framework and
simply use generic component framework also in G2D driver.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>