meta: Rearrange shader creation in setup_glsl_generate_mipmap

meta: Rearrange shader creation in setup_glsl_generate_mipmap

The diff looks weird, but this moves the code from the first 'if
(ctx->Const.GLSLVersion < 130)' block down into the second block.  It
also moves some variable decalarations closer to their use.

NOTE: This is a candidate for the 9.0 branch.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>