drm/exynos: remove duplicated error routine and unnecessary assign

There were duplicated error handling routines during allocating
pages in lowlevel_buffer_allocate() and g2d_userptr_get_dma_addr().
Also unnecessary NULL assignments for variable used not any more
are removed from g2d_userptr_get_dma_addr() and
g2d_userptr_put_dma_addr().

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