drm/exynos: use __free_page() to deallocate memory

this patch uses __free_page() to deallocate the pages allocated
by alloc_page() and the pages doesn't need set_parge_dirty()
and mark_page_accessed() because they aren't from page cache so
removes them.

this patch has a pair with previous patch below,
	http://www.spinics.net/lists/dri-devel/msg24382.html



Changelog v2:
remove unnecessary arguments.

Changelog v3:
fix npages type.
- npages can have negative value.

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