panfrost: Fixup vertex offsets to prevent shadow copy

panfrost: Fixup vertex offsets to prevent shadow copy

Mali attribute buffers have to be 64-byte aligned. However, Gallium
enforces no such requirement; for unaligned buffers, we were previously
forced to create a shadow copy (slow!). To prevent this, we instead use
the offseted buffer's address with the lower bits masked off, and then
add those masked off bits to the src_offset. Proof of correctness
included, possibly for the opportunity to say "QED" unironically.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>