drm/exynos: use DMA_ATTR_NO_KERNEL_MAPPING attribute

Changelog v3:
just code cleanup.

Changelog v2:
fix argument to dma_mmap_attr function.
- use pages instead of kvaddr because kvaddr is 0 with
  DMA_ATTR_NO_KERNEL_MAPPING.

Changelog v1:
When gem allocation is requested, kernel space mapping isn't needed.
But if need, such as console framebuffer, the physical pages would be
mapped with kernel space though vmap function.

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