anv/descriptor_set: Only write texture swizzles if we have an image view
When immutable samplers are set we call write_image_view with a NULL image view. This causes issues on IVB where we have to fake texture swizzling. Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110999 Fixes: d2aa65eb "anv: Emulate texture swizzle in the shader when..." (cherry picked from commit 0a364a4a)