drm/xe: Apply upper limit to sg element size

The iommu_dma_map_sg() function ensures iova allocation doesn't
cross dma segment boundary. It does so by padding some sg elements.
This can cause overflow, ending up with sg->length being set to 0.
Avoid this by halving the maximum segment size (rounded down to
PAGE_SIZE).

Specify maximum segment size for sg elements by using
sg_alloc_table_from_pages_segment() to allocate sg_table.

v2: Use correct max segment size in dma_set_max_seg_size() call

Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Reviewed-by: Bruce Chang <yu.bruce.chang@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>