winsys/svga: Limit the maximum DMA hardware buffer size

The kernel total GMR/DMA size is limited, but it's definitely possible for the
kernel to allow a larger buffer allocation to succeed, but command
submission using that buffer as a GMR would fail typically causing an
application crash.

So have the winsys limit the size of GMR/DMA buffers. The pipe driver will
then resort to allocating smaller buffers and perform the DMA transfer in
multiple bands, also allowing for the pre-flush mechanism to kick in.

This avoids the related application crashes.

Fixes: e7843273 ("winsys/svga: Update to vmwgfx kernel module 2.1")
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
(cherry picked from commit 91146c07)