radv/rt: Use ushr for extracting the cull mask
Fixes the following tests:
dEQP-VK.ray_tracing_pipeline.acceleration_structures.ray_cull_mask.gpu_built.ahit.4_bits
dEQP-VK.ray_tracing_pipeline.acceleration_structures.ray_cull_mask.gpu_built.ahit.16_bits
dEQP-VK.ray_tracing_pipeline.acceleration_structures.ray_cull_mask.gpu_built.chit.4_bits
dEQP-VK.ray_tracing_pipeline.acceleration_structures.ray_cull_mask.gpu_built.chit.16_bits
dEQP-VK.ray_tracing_pipeline.acceleration_structures.ray_cull_mask.gpu_built.isec.4_bits
dEQP-VK.ray_tracing_pipeline.acceleration_structures.ray_cull_mask.gpu_built.isec.16_bits
Fixes: 2d93ab7 ("radv/rt: Pre shift cull_mask")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21659>