Redesign DumbBuf

[trissylegs]

Currently DumbBuf is one big opaque blob (with a GemHandle implemntation).
Which was mostly done to Just Get It Working™.

The steps for using a dumb buf and mapping it are:

1. Create Dumb buffer.
2. create something that can be mapped.
   a. Use map dump which gives you an offset.
   b. Create a PRIME fd
3. mmap
4. Create a Drm Fb.

Currently it:

1. Creates the buffer
2. Dups device
3. Creates the offset,
4. Mmaps it.
5. Then returns all that in one big mess (with a generic parameter)

I would like something that looks like:

let dumb = device.create_dumb::<Argb8888>(width, height);
let prime = dumb.create_prime();
let map = prime.map();

Where prime is some struct Prime. create_prime is actaully from a Gem trait. (Or GemExt for AsRef<&GemHandle>), and Argb8888 is some pixel type. (We have those already) with a trait like so:

pub trait PixelFormat {
    type Pixel: Copy, Sized, From<u32>;
    fn buffer_as(buf: &[u8]) -> &[Self::Pixel];
}

map is probably going to be some wrapper around Memmap.