i965/fs: initialize src as reg_undef for texture opcodes on Gen5/6.

i965/fs: initialize src as reg_undef for texture opcodes on Gen5/6.

Commit 07af0ab changed fs_inst to have 0 sources for texture opcodes
in emit_texture_gen5 (Ironlake, Sandybrige) while fs_generator still
uses a single source from brw_reg struct. Patch sets src as reg_undef
which matches the behavior before the constructor got changed.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=79534