vulkan: don't hold the device mutex while compiling pipelines (llama/23641)

vulkan: don't hold the device mutex while compiling pipelines (llama/23641)

* vulkan: don't hold the device mutex while compiling pipelines

We need to hold a lock while we traverse all pipelines and lazily initialize
them, but we don't need to hold it while the pipeline is being compiled. And
it doesn't need to be the same lock as the device mutex. We call load_shaders
each time a pipeline is needed, so we only need to compile that one pipeline
(and, for example, don't want to end up compiling a pipeline that another
thread should be compiling).

* remove 'needed'