glsl: Fix ast-to-hir for ARB_fragment_coord_conventions

glsl: Fix ast-to-hir for ARB_fragment_coord_conventions

Function ast_declarator_list::hir(), when processing keywords added by
extension ARB_fragment_coord_conventions, made the mistake of checking only if
the extension was __supported by the driver__. The correct behavior is to check
if the extensi0n is __enabled in the parse state__.

NOTE: this is a candidate for the 7.9 branch.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>