gallium/aux/tgsi/tgsi_dump.c: Fix -Wsign-compare warnings

gallium/aux/tgsi/tgsi_dump.c: Fix -Wsign-compare warnings

tgsi_dump.c: In function 'iter_property':
tgsi_dump.c:443:18: warning: comparison between signed and unsigned
integer expressions [-Wsign-compare]
    for (i = 0; i < prop->Property.NrTokens - 1; ++i) {
                  ^
tgsi_dump.c:459:13: warning: comparison between signed and unsigned
integer expressions [-Wsign-compare]
       if (i < prop->Property.NrTokens - 2)

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>