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

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

tgsi/tgsi_lowering.c: In function 'emit_twoside':
tgsi/tgsi_lowering.c:1179:18: warning: comparison between signed and
unsigned integer expressions [-Wsign-compare]
    for (i = 0; i < ctx->two_side_colors; i++) {
                  ^
tgsi/tgsi_lowering.c:1208:18: warning: comparison between signed and
unsigned integer expressions [-Wsign-compare]
    for (i = 0; i < ctx->two_side_colors; i++) {
                  ^
tgsi/tgsi_lowering.c:1216:18: warning: comparison between signed and
unsigned integer expressions [-Wsign-compare]
    for (i = 0; i < ctx->two_side_colors; i++) {
                  ^
tgsi/tgsi_lowering.c: In function 'emit_decls':
tgsi/tgsi_lowering.c:1280:18: warning: comparison between signed and
unsigned integer expressions [-Wsign-compare]
    for (i = 0; i < ctx->numtmp; i++) {
                  ^
tgsi/tgsi_lowering.c: In function 'rename_color_inputs':
tgsi/tgsi_lowering.c:1311:28: warning: comparison between signed and
unsigned integer expressions [-Wsign-compare]
             if (src->Index == ctx->two_side_idx[j]) {
                            ^~

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