drm/prime: replace NULL with error value in drm_prime_pages_to_sg

Instead of NULL, error value is casted with ERR_PTR() for
drm_prime_pages_to_sg() and IS_ERR_OR_NULL() macro is replaced
with IS_ERR() macro for drm_gem_map_dma_buf().

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: Dave Airlie <airlied@redhat.com>