tu: Return the correct alignment for images

Connor Abbott requested to merge cwabbott0/mesa:review/tu-image-align into master

This fixes some dEQP-VK.binding_model.descriptorset_random failures, which happened when the test tried to allocate (I think) an array of images, and used the alignment of 0 we returned to compute a size of 0 and things exploded. The blob driver seems to be lazy and over-specifies the alignment, so I determined the actual alignments by hacking amber and later the dEQP pipeline.sampler tests to allocate images with an offset and then observing the smallest offset where things didn't fail.

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