svga: fix incorrect user buffer size computation

svga: fix incorrect user buffer size computation

Viewperf uses some unusual vertex arrays where the stride is less
than the element size.  In this case, the stride was 4 while the
element size was 12.  The difference of 8 bytes causes us to miss
uploading the tail bit of the array data.

Typically the stride is >= the element size so there was no problem
with other apps.