Dirty draw buffers are causing cache clears on Manhattan.
Instead of clearing out the entire FramebufferVk cache when draw
buffers are dirty, reset the cache signature and regenerate it.

This change also contains a fix to make sure that we're not generating
serials for a draw buffer that's disabled.

Thanks Jamie Madill!

Bug: angleproject:4442
Change-Id: I0d48a2d6d95e74898a11bdde0fedbce77c82a3a3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2091862Reviewed-by: Tobin Ehlis <tobine@google.com>
Commit-Queue: Tobin Ehlis <tobine@google.com>