v3dv/pipeline: add support for shader variants

v3dv/pipeline: add support for shader variants

So far, we were doing the compilation to qpu when the pipeline was
created (as part of vkCreateGraphicsPipeline).

But this would not be correct when some specific descriptors are
involved, like textures. For that case some nir lowerings depend on
the texture format, and that info is not available until the specific
descriptors are bound to the command buffer. In the same way, the same
command buffer with a given pipeline could get their descriptor bound
again.

So it would be needed to support compilation variants of the same
shader. So finally, the v3d_key would work as keys, as the variants
would be tracked with a hash table.

This commit introduces the new structures for that. What we were
building as the final qpu shader would become the initial default
variant for the pipeline. We are also saving the keys used at that
point, to avoid needing to fully regenerate them when a new variant is
created. Not just for performance, but also to avoid needing to track
the graphics pipeline create info structure.

The code to handle updating the current variant would be done on
following commits.

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