pvr: Fix dynamic desc offset storage

pvr: Fix dynamic desc offset storage

The index at which the dynamic descriptor offsets were being
stored was incorrect, leading to some offsets not being stored and
thus `0` being applied as the offset to the descriptors instead.

dEQP test fixed:
  dEQP-VK.binding_model.shader_access.{primary,secondary}_cmd_buf
    .uniform_buffer_dynamic
    .{vertex,fragment,compute,vertex_fragment}
    .multiple_discontiguous_descriptor_sets
    .*_descriptor.offset_view_{,non}zero_dynamic_nonzero

Fixes: aa791961a82e ("pvr: Add support for dynamic buffers descriptors")
Signed-off-by: Karmjit Mahil <Karmjit.Mahil@imgtec.com>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25320>