v3dv/pipeline_cache: extend pipeline cache envvar

v3dv/pipeline_cache: extend pipeline cache envvar

So far V3DV_ENABLE_DEFAULT_PIPELINE_CACHE allowed to configure
pipeline cache to avoid any caching using a pipeline cache.

With this change we can be more detailed. Then envvar is not anymore a
boolean. Allowed values:

  * "off": no pipeline cache at all. PipelineCache objects behaves as
    no-op objects.

  * "no-default-cache": user PipelineCache caches nir/variants, but we
    don't provide a default cache in case the user doesn't provide a
    PipelineCache object, neither for internal pipelines.

  * "full" (default): we provide a default PipelineCache, used when
    the user doesn't provide one when creating a Pipeline, and for
    internal Pipelines.

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