drm/exynos: simplify Kconfig component names

Many Exynos DRM sub-options mentions Exynos DRM in their titles.
It is redundant and can be safely shortened. The patch additionally
makes some entries more descriptive.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>