When the local types used to walk the DBE request were changed, this
changed the type of the parameter passed to the DDX SwapBuffers API,
but there wasn't a matching change in the API definition.

At this point, with the API frozen, I just stuck a new variable in
with the correct type. Because we've already bounds-checked nStuff to
be smaller than UINT32_MAX / sizeof(DbeSwapInfoRec), we know it will
fit in a signed int without overflow.

Signed-off-by: Keith Packard <keithp@keithp.com
Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Backported-to-NX-by: Ulrich Sibiller <uli42@gmx.de>