v3dv/pipeline: when looking for a variant, check first current variant

v3dv/pipeline: when looking for a variant, check first current variant

So far, when checking for a variant fulfilling a specific v3d key, we
were checking the caches, and if that failed, we compiled a new
variant, and update the current variant.

But we could check first if the current variant fullfils that. This
was not really problematic so far, as checking on the caches was fast,
but now that we could be without any kind of shader cache using
V3DV_ENABLE_PIPELINE_CACHE, it is far better to check first current
variant.

Without this vkQuake3 at 720p drops to 1fps when disabling the cache.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6766>