vkr: require render server for venus and clean up iovs

For a secure integration, venus must be shipped with process isolation in production, since vulkan cmd validation has non-trivial overhead even with minimal validation while itself is also incomplete in coverage even with full validation. This MR enforces that and cleans up the unused iov codes that can break sooner or later since unused in production and untested.

• 1st commit updates meson to require render server for venus
• 2nd commit is mainly a revert of the original large ring support in !603 (merged), but keeps all the good refactorings
• 3~5 commits clean up the legacy iov support from the command stream, cs encoder and lastly resource attachment. It also allows to avoid the extra alloc/copy of the command stream since decode always sees contiguous data.
• last commit further cleans up some tiny bits as a wrap up

Test:

• dEQP-VK
• vtest