vc4: pre-compile shaders to make up for the lack of draw calls in shader-db

vc4: pre-compile shaders to make up for the lack of draw calls in shader-db

This means we don't have the variant keys, and need to make up one
variant and pre-compile it.

Signed-off-by: Eric Engestrom <eric@igalia.com>
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18599>