pvr: Convert attachment indexes to unsigned.

pvr: Convert attachment indexes to unsigned.

This commit converts attachment indexes used in renderpass to unsigned
and uses VK_ATTACHMENT_UNUSED for unused attachments instead of -1.

Also fixes some signed to unsigned comparisons.

Fixed: CID 1515587, 1515590, 1515597, 1515604.

Signed-off-by: Rajnesh Kanwal <rajnesh.kanwal@imgtec.com>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Reviewed-by: Karmjit Mahil <Karmjit.Mahil@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18866>