glsl: Set the uniform_block index for the linked shader variables.

glsl: Set the uniform_block index for the linked shader variables.

At this point in the linking, we've totally lost track of the struct
gl_uniform_buffer that this pointed to in the original unlinked
shader, so we do a nasty n^2 walk to find it the new one based on the
variable name.

Note that these point into the shader's list of gl_uniform_buffers,
not the linked program's.

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