glsl: Initialize ir_variable_data::fb_fetch_output earlier for GL(ES) 2.

glsl: Initialize ir_variable_data::fb_fetch_output earlier for GL(ES) 2.

At the same point where it is initialized on GL(ES) 3.0+ so we can
implement some common layout qualifier handling in a future commit.
Until now the fb_fetch_output flag would be inherited from the
original implicit gl_LastFragData declaration at a later point in the
AST to GLSL IR translation.

Reviewed-by: Plamena Manolova <plamena.manolova@intel.com>