sna: Allow ridiculously large bo, up to half the total GATT

Such large bo place extreme stress on the system, for example trying to
mmap a 1GiB into the CPU domain currently fails due to a kernel bug. :(
So if you can avoid the swap thrashing during the upload, the ddx can now
handle 16k x 16k images on gen4+ on the GPU. That is fine until you want
two such images...

The real complication comes in uploading (and downloading) from such
large textures as they are too large for a single operation with
automatic detiling via either the BLT or the RENDER ring. We could do
manual tiling/switching or, as this patch does, tile the transfer in
chunks small enough to fit into either pipeline.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

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