panfrost: Compute support, Midgard part 1

These commits are the first batch of changes from my cl branch, with just the non-WIP commits that apply to Midgard.

The changes aren't that useful on their own, as pipe_kmsro.so is still broken and Clover uses set_shader_images which Panfrost doesn't yet define, but merging them now will mean there won't be quite so many commits to deal with later on.