drm/radeon: fix scatter-gather mapping with user pages

Calls to dma_map_sg may return less segments / entries than requested
if they fall on page bounderies. The old implementation did not
support this use case.

Fixes: be62dbf5 ("iommu/amd: Convert AMD iommu driver to the dma-iommu api")
Bug: https://bugzilla.kernel.org/show_bug.cgi?id=206461
Bug: https://bugzilla.kernel.org/show_bug.cgi?id=206895
Bug: https://gitlab.freedesktop.org/drm/amd/issues/1056


Signed-off-by: Shane Francis <bigbeeshane@gmail.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200325090741.21957-4-bigbeeshane@gmail.com
Cc: stable@vger.kernel.org