iris: Handle PIPE_TRANSFER_DISCARD_WHOLE_RESOURCE somewhat

iris: Handle PIPE_TRANSFER_DISCARD_WHOLE_RESOURCE somewhat

Various places in the transfer code need to know whether they must
read the existing resource's values.  Rather than checking both flags
everywhere, just make PIPE_TRANSFER_DISCARD_WHOLE_RESOURCE also flag
PIPE_TRANSFER_DISCARD_RANGE - if we can discard everything, we can
discard a subrange, too.

Obviously, we can do better for PIPE_TRANSFER_DISCARD_WHOLE_RESOURCE,
but eventually u_threaded_context should handle swapping out buffers
for new idle buffers, anyway.  In the meantime, this is at least better.