vk: Re-name command buffer implementation files

vk: Re-name command buffer implementation files

Previously, the command buffer implementation was split between
anv_cmd_buffer.c and anv_cmd_emit.c.  However, this naming convention was
confusing because none of the Vulkan entrypoints for anv_cmd_buffer were
actually in anv_cmd_buffer.c.  This changes it so that anv_cmd_buffer.c is
what you think it is and the internals are in anv_batch_chain.c.