freedreno/drm/sp: Implement deferred submit merging

freedreno/drm/sp: Implement deferred submit merging

For submits flushed with (a) no required fence, and (b) no externally
visible effects (ie. imported/exported bo), we can defer flushing the
submit and merge it into a later submit.

This is a bit more work in userspace, but it cuts down the number of
submit ioctls.  And a common case is that later submits overlap in the
bo's used (for example, blit upload to a buffer, which is then used in
the following draw pass), so it reduces the net amount of work needed
to be done in the kernel to handle the submit ioctl.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/19
Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10444>