vulkan/overlay: fix command buffer stats

Begin/Reset of command buffer both reset the content of the command
buffer. Don't forget to wipe them on Begin.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 4438188f ("vulkan/overlay: record stats in command buffers and accumulate on exec/submit")
Acked-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit 8f0f727f)