glsl: Don't look at sampler uniform storage for internal vars

glsl: Don't look at sampler uniform storage for internal vars

Passes like nir_lower_drawpixels add additional sampler variables,
and set an explicit binding which never changes.  These extra samplers
don't have proper uniform storage associated with them, and there is no
way to update bindings via the API.  So, for any 'hidden' variables,
just trust that there's an explicit binding set.

Reviewed-by: Eric Anholt <eric@anholt.net>