tgsi: add support for buffer/atomic operations to tgsi_exec

[Dieter Nützel]

Warning during compilation after git pull:

tgsi/tgsi_exec.c: In function 'exec_atomop_buf':
tgsi/tgsi_exec.c:4060:8: warning: array subscript is above array bounds 
[-Warray-bounds]
        r[3].f[j] = rgba[3][j];
         ^

Any hints where I should look?

Dieter