r600g: Ensure r600_src is initialized in tgsi_exp function.

r600g: Ensure r600_src is initialized in tgsi_exp function.

Silences these GCC warnings.
r600_shader.c: In function 'tgsi_exp':
r600_shader.c:2339: warning: 'r600_src[0].rel' is used uninitialized in this function
r600_shader.c:2339: warning: 'r600_src[0].abs' is used uninitialized in this function
r600_shader.c:2339: warning: 'r600_src[0].neg' is used uninitialized in this function
r600_shader.c:2339: warning: 'r600_src[0].chan' is used uninitialized in this function
r600_shader.c:2339: warning: 'r600_src[0].sel' is used uninitialized in this function