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.