gallium/aux/tgsi_build.c: Remove unused parameters prev_token from various functions

gallium/aux/tgsi_build.c: Remove unused parameters prev_token from various functions

remove parameter prev_token unused in
   tgsi_build_instruction_label
   tgsi_build_instruction_texture
   tgsi_build_instruction_memory
   tgsi_build_texture_offset

This fixes the following warnings:

tgsi/tgsi_build.c: In function 'tgsi_build_instruction_label':
tgsi/tgsi_build.c:716:24: warning: unused parameter 'prev_token' [-
Wunused-parameter]
    struct tgsi_token  *prev_token,
                        ^~~~~~~~~~
tgsi/tgsi_build.c: In function 'tgsi_build_instruction_texture':
tgsi/tgsi_build.c:749:23: warning: unused parameter 'prev_token' [-
Wunused-parameter]
    struct tgsi_token *prev_token,
                       ^~~~~~~~~~
tgsi/tgsi_build.c: In function 'tgsi_build_instruction_memory':
tgsi/tgsi_build.c:784:23: warning: unused parameter 'prev_token' [-
Wunused-parameter]
    struct tgsi_token *prev_token,
                       ^~~~~~~~~~
tgsi/tgsi_build.c: In function 'tgsi_build_texture_offset':
tgsi/tgsi_build.c:819:23: warning: unused parameter 'prev_token' [-
Wunused-parameter]
    struct tgsi_token *prev_token,
                       ^~~~~~~~~~

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