anv: end command buffer with a potential jump

anv: end command buffer with a potential jump

We would like to chain multiple primary command buffer to be submitted
together to i915. For prepare this, add end the command buffers with a
MI_BATCH_BUFFER_START and at submit time, replace it with
MI_BATHC_BUFFER_END if needed.

v2: Don't even consider non softpin platforms

v3: Fix inverted condition

v4: Limit is_chainable() to checking device->use_softpin (Jason)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/2371>