mesa: don't build llvmpipe when ~llvm

[wdconinc]

This PR modifies mesa to reflect https://gitlab.freedesktop.org/mesa/mesa/-/commit/010b2f9497a. swrast was (removed since then) an alias for softpipe and llvmpipe. When ~llvm, the configuration with llvmpipe fails (see e.g. #824 (comment)).

Note: There may be other ways to support this (e.g. instead of adding swrast by default, we could add both llvmpipe and softpipe explicitly), but I'm trying to apply a minimal fix to the issue at hand.

[Copilot]

Pull request overview

This PR addresses a build failure in mesa when built without LLVM support by conditionally selecting gallium drivers based on the llvm variant. The change reflects an upstream mesa commit (010b2f9497a) that clarifies the relationship between swrast, softpipe, and llvmpipe drivers.

Changes:

• Adds conditional logic to select "swrast" gallium drivers when +llvm variant is enabled
• Uses "softpipe" alone when ~llvm variant is specified (since llvmpipe requires LLVM)
• Adds a comment explaining that swrast includes both softpipe and llvmpipe

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