panfrost: Simplify Bifrost blendable formats

Delete a lot of indirection. @icecream95 proposed a fast path for rgba8_unorm but if we can make 1 format fast, why not make all the formats fast?

Running on dEQP but haven't profiled anything. Then again the code cleanup alone is worth it imo.

Also seriously considering deleting support for non-dithered tib formats, we don't use them and I don't get when you'd want to. There's already a mechanism outside this to implement GL_DITHER. Maybe useful for Vulkan?