gallium/aux/tgsi_build.c: Fix -Wsign-compare warnings

gallium/aux/tgsi_build.c: Fix -Wsign-compare warnings

tgsi/tgsi_build.c: In function 'tgsi_build_full_immediate':
tgsi/tgsi_build.c:622:18: warning: comparison between signed and
unsigned integer expressions [-Wsign-compare]
    for( i = 0; i < full_imm->Immediate.NrTokens - 1; i++ ) {
                  ^
tgsi/tgsi_build.c: In function 'tgsi_build_full_property':
tgsi/tgsi_build.c:1393:18: warning: comparison between signed and
unsigned integer expressions [-Wsign-compare]
    for( i = 0; i < full_prop->Property.NrTokens - 1; i++ ) {
                  ^

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