pvr: Fix pvr_csb_bake() list return.

pvr: Fix pvr_csb_bake() list return.

The list logic checks for list->next->next (+ some other checks)
to point to the list itself to determine that there is just one
single element.

 ┌───────────────────────┐
 └< { HEAD } >─< { E0 } >┘

When the list_head is copied as was being done previously, the
list element's next pointer still points at the old head so
the `list_is_singular()` check fails.

Fixes pvr_cmd_buffer.c:605:`list_is_singular(&bo_list)` assertion
dEQP-VK.api.image_g.core.clear_color_attachment.cube_layers.b8g8r8a8_unorm

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/22730>