glsl: Create and use a has_uniform_buffer_objects() helper.

glsl: Create and use a has_uniform_buffer_objects() helper.

This is better than overriding the extension enable based on the
language version; it's robust against shaders that do:

   #version 140
   #extension GL_ARB_uniform_buffer_object : disable

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>