`cuInit` wakes up the nvidia dgpu on nvidia laptops. This is bad news because the wake up process
is blocking and takes a few seconds. It also needlessly increases power consumption.

Sometimes, a VO loads several hwdecs (like `dmabuf_wayland`). When `cuda` is loaded, it calls
`cuInit` before running all interop inits. However, the first checks in the interops do not
require cuda initialization, so we only need to call `cuInit` after those checks.

This commit splits the interop `init` function into `check` and `init`. `check` can be called without
initializing the Cuda backend, so cuInit is only called *after* the first interop check.

With these changes, there's no cuda initialization if no OpenGL/Vulkan backend is available. This prevents
`dmabuf_wayland` and other VOs which automatically load cuda from waking up the nvidia dgpu unnecessarily,
making them start faster and decreasing power consumption on laptops.

Fixes: mpv-player#13668

This simplifies the code and makes it easier to read.