nir/linker: handle uniforms without explicit location

nir/linker: handle uniforms without explicit location

ARB_gl_spirv points that uniforms in general need explicit
location. But there are still some cases of uniforms without location,
like for example uniform atomic counters. Those doesn't have a
location from the OpenGL point of view (they are identified with a
binding and offset), but Mesa internally assigns it a location.

Signed-off-by: Eduardo Lima <elima@igalia.com>
Signed-off-by: Alejandro Piñeiro <apinheiro@igalia.com>
Signed-off-by: Neil Roberts <nroberts@igalia.com>
v2: squash with another patch, minor variable name tweak (Timothy
Arceri)

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>