vrend: Make sure there are no interger overflows in size calculation

Since the final value is uint64_t we can also make some  intermediate
value uint64_t so that multiliocations can't overflow.

Fixes Coverity IDs: 1527232, 1527196,1527197
     "Unintentional integer overflow"

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/virgl/virglrenderer/-/merge_requests/975>
1 file changed