drm/gpusvm: keep an IOVA mapped range dma address inline

dma_iova_try_alloc() reserves one contiguous IOVA for the whole range and
links each page at the next offset, so the device addresses run
contiguously from entry 0 and one entry describes them all. A 2 MiB range
of 4 KiB pages then drops the same 8 KiB array as a THP backed one.

Fold only when state_offset covers the full range, which proves no device
page was mapped in between, and only single page entries, so the order
kept is 0 and stays true. Widening it instead would tell a consumer to use
a huge page for npages separate CPU pages, which hangs Vega20 on amdgpu.

The kept entry no longer bounds the segment, so skip the unmap walk when
it has nothing to do, keyed off dpagemap rather than the flags, which are
not published yet on the error unwind. Consumers need the same
distinction, so drm_gpusvm_pages_first_dma() returns it alongside the
array from one read of the flags; xe passes it to xe_res_first_dma().

Suggested-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Honglei Huang <honghuan@amd.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20260905133142.3628027-7-honghuan@amd.com

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