asahi: unbind samplers and fix sampler_count if state is NULL
When states is NULL, unbind samplers (to avoid dangling pointers) and set sampler_count to the highest non-null samplers[] entry instead of setting it to 0.
This is ported from a similar fix in panfrost: mesa/mesa!20285 (merged)
Signed-off-by: Iago López Galeiras iaguis@gmail.com