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

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

tgsi/tgsi_text.c: In function 'parse_identifier':
tgsi/tgsi_text.c:218:16: warning: comparison between signed and unsigned
integer expressions [-Wsign-compare]
          if (i == len - 1)
                ^~
tgsi/tgsi_text.c: In function 'parse_optional_swizzle':
tgsi/tgsi_text.c:873:21: warning: comparison between signed and unsigned
integer expressions [-Wsign-compare]
       for (i = 0; i < components; i++) {
                     ^
tgsi/tgsi_text.c: In function 'parse_instruction':
tgsi/tgsi_text.c:1103:18: warning: comparison between signed and
unsigned integer expressions [-Wsign-compare]
    for (i = 0; i < info->num_dst + info->num_src + info->is_tex; i++) {
                  ^
tgsi/tgsi_text.c:1118:18: warning: comparison between signed and
unsigned integer expressions [-Wsign-compare]
       else if (i < info->num_dst + info->num_src) {
                  ^
tgsi/tgsi_text.c: In function 'parse_immediate':
tgsi/tgsi_text.c:1660:24: warning: comparison between signed and
unsigned integer expressions [-Wsign-compare]
    for (type = 0; type < ARRAY_SIZE(tgsi_immediate_type_names); ++type)
{
                        ^
Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>