panfrost: Only link varyings once in good conditions

If the varying descriptors will always be the same for a given shader variant (certainly true if none of separable shaders, transform feedback, or point sprites are used), we only need to link once. Now that pan_pool supports both owned and unowned modes, we have the flexibility to reuse the code path for both allocation strategies.

Signed-off-by: Alyssa Rosenzweig alyssa@collabora.com