drm/prime: fix extracting of the DMA addresses from a scatterlist

Scatterlist elements contains both pages and DMA addresses, but one
should not assume 1:1 relation between them. The sg->length is the size
of the physical memory chunk described by the sg->page, while
sg_dma_len(sg) is the size of the DMA (IO virtual) chunk described by
the sg_dma_address(sg).

The proper way of extracting both: pages and DMA addresses of the whole
buffer described by a scatterlist it to iterate independently over the
sg->pages/sg->length and sg_dma_address(sg)/sg_dma_len(sg) entries.

Fixes: 42e67b47 ("drm/prime: use dma length macro when mapping sg")
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200327162126.29705-1-m.szyprowski@samsung.com
Cc: stable@vger.kernel.org