panfrost: Preliminary work for mipmaps

panfrost: Preliminary work for mipmaps

This patch refactors a substantial amount of code in preparation for
mipmaps. In particular, we know have a correct slice abstraction based
on offsets; cpu/gpu are no longer arbitrary pointers. We additionally
shuffle around other code to accompany these changes and cleanup how
tiled textures are handled, while drawing some attention to the blit
code.

Mipmaps are still disabled at this point, as autogeneration is not yet
implemented; enabling as-is would cause regressions.

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