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