drm/exynos: fix allocation and cache mapping type

This patch fixes memory alloction(contiguous or not) and
cache mapping types(cachable or not).
For this, it converts each type from user request into dma
attribute properly.

Changelog v2:
- just code cleanup.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>