pvr: Fix segfaults when pDepthStencilAttachment is NULL

pvr: Fix segfaults when pDepthStencilAttachment is NULL

depth_stencil_attachment has been changed from a pointer to the attachment idx
to just the attachment idx, as this avoids the driver having to check for NULL
when comparing attachments indexes with depth_stencil_attachment.

Anyplace that relies on depth_stencil_attachment being a valid index must
already check that depth_stencil_attachment is not VK_ATTACHMENT_UNUSED, so
this change avoids having to check both the pointer and the index for the same
information.

Noticed when running dEQP-VK.api.smoke.triangle

Signed-off-by: Jarred Davies <jarred.davies@imgtec.com>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21690>