v3dv: one step pipeline cache

Right now we are doing always a two-step cache lookup. First we search the nir shaders on the cache, and then the variants. This is done this way because some of the fields of the v3d key depends on values of the nir shaders, or on additional info that the store on the pipeline but gather during nir lowerings.

But after a review, all that info is implicit on the original SPIR-V shader (or internal NIR shaders), so it would be safe to generate a pipeline cache key without that info, and try to get directly the shader variant.