x86/dma-debug: Bump PREALLOC_DMA_DEBUG_ENTRIES

I ran out of free entries when I had CONFIG_DMA_API_DEBUG
enabled. Some other archs seem to default to 65536, so increase
this limit for x86 too.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Link: http://lkml.kernel.org/r/50A612AA.7040206@canonical.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
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