glsl: fix stream qualifier for blocks with an instance name

glsl: fix stream qualifier for blocks with an instance name

This also removes the validation from the parser as it is not required
and once arb_enhanced_layouts comes along we wont be able to do validation
on the stream qualifier in the parser anyway as it adds constant expression
support to the stream qualifier.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Cc: 11.0 <mesa-stable@lists.freedesktop.org>