radv: replace an assertion with a conditional

radv: replace an assertion with a conditional

Replace the !binding_layout->immutable_samplers assertion in
radv_update_descriptor_sets with a conditional.

The Vulkan specification does not say that it is illegal to update
a sampler descriptor when it is immutable; only that pImageInfo is
ignored.

This change is also needed for push descriptors, because valid
descriptors must be pushed for all bindings accessed by shaders,
including immutable sampler descriptors.

Signed-off-by: Fredrik Höglund <fredrik@kde.org>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>