drm/exynos: fix pages allocation size in lowlevel_buffer_allocate

When IOMMU is not supported, buf->pages has to be allocated to
assign the result of phys_to_page() which return type is struct
page *. So it is sufficient to allocate buf->pages with the size
of multiple struct page pointers.

Signed-off-by: YoungJun Cho <yj44.cho@samsung.com>
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>