Add radeonsi to the drivers being build
This MR enables building of the radeonsi driver and removes the explicit variables forcing swrast.
With a HW enabled runner this will result in Xvfb, apitrace and renderdoc picking up GPU and producing the appropriate results. For example see this run.
Note: running this patch w/o the corresponding images in the database (MR opened here) will result in a crash. Latter of which is addressed with this MR
In practise Mesa will fall-back to swrast, when GPU is not present. Although I've not tested it against all our shared runners. As a follow-up we should look into tagging the runners as applicable, to avoid potential problems.
UPDATE: the runtime driver selection/handling is tracked in separate MR !22 (merged). Everything else is still applicable.