turnip, freedreno: Move image view setup to fdl

To implement VK_KHR_maintenance4 we'll have to start deriving tu_image_view from vk_image_view, which contains a bunch of Vulkan-specific things like a Vulkan format. However in our blit code we need to create a view of a staging resource with a "Y8" format that doesn't exist in Vulkan, so we need a way to create a view with only a pipe_format that may not correspond to a VkFormat. Stuck between a rock and a hard place, this series takes a page from isl and its isl_view structure, and moves the guts of tu_image_view into a Vulkan-independent fdl6_view struct. We can then start passing this struct around in our copy/blit code instead of tu_image_view. As a bonus, we'll hopefully be able to use this from freedreno in the future. (This seems a little tricky, because Gallium has two different view types for textures and images vs. just one in Vulkan, but that's left for future work.)