panfrost: fix sampler_count and unbind samplers in bind_sampler_states

panfrost: fix sampler_count and unbind samplers in bind_sampler_states

1. Old approach did not support unbind (set to NULL) samplers because
it only copied memory if sampler is not empty. New approach checks
if sampler is empty - it will set NULL.

2. Old approach just set sampler_count to 0 if sampler is empty.
That's wrong and we need to find highest non-null samplers[] entry.
It was done in new approach.

3. Gallium dosc says:
```
NOTE: at this time, start is always zero ...
This may change in the future.
```
It's better to take into consideration start parameter in new approach.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20285>