asahi: Wire up compute kernels

asahi: Wire up compute kernels

Now that we have multiple sysval tables, implementing compute kernels --
including with indirect dispatch and load_num_workgroups -- is straightforward.
This patch adds the straightforward launch_grid implementation.

As usual needs UAPI support patches to actually do anything, but the relevant
compute tests are passing downstream.

It's not possible to properly test compute shaders support right now (pending
support for images), so we don't update the CAPs or features.txt here. This is
more about flushing out the piles of downstream patches we have (and getting
reviewed!) in preparation for cutting a downstream release soon.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21703>