vulkan/runtime: don't lookup the pipeline disk cache if disabled

vulkan/runtime: don't lookup the pipeline disk cache if disabled

When the Anv pipeline got migrated to the runtime, we gain/lost a bit
of functionality which is that the disk cache is always read
regardless of VK_ENABLE_PIPELINE_CACHE=0.

This change brings the old behavior back.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 591da9877900c ("vulkan: Add a common VkPipelineCache implementation")
Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19050>