radv: always write the sample positions when a new descriptor BO is created

radv: always write the sample positions when a new descriptor BO is created

This was completely broken, for example in the following scenario:
- submits something which needs sample positions (this creates a new
descriptor BO with sample positions)
- submits something which needs the tess rings (this creates a new
descriptor BO with tess rings but without sample positions, ie.
add_sample_positions would be FALSE)
- submits something which needs sample positions again (this won't
create a new descriptor BO because it incorrectly remembered that
sample positions were set)

Fix this by always writing the sample positions.

This should fix the following flakes:
- dEQP-VK.fragment_shading_barycentric.*.weights.pipeline_topology_dynamic.msaa_interpolate_at_sample.*
- dEQP-VK.pipeline.fast_linked_library.multisample_interpolation.sample_interpolate_at_distinct_values.*
- dEQP-VK.pipeline.fast_linked_library.multisample_interpolation.sample_interpolation_consistency.*
- dEQP-VK.draw.renderpass.linear_interpolation.*
- dEQP-VK.draw.dynamic_rendering.primary_cmd_buff.linear_interpolation.*

These flakes were extremely hard to reproduce!

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25529>