[RFC] panfrost: Replace access->{readers,writer} by access->users

We get rid of the writer/has_writer/readers field and replace it by
a users bitmap. This means the access tracker now has to go check the
type of access by dereferencing the batch->bos array when the number
of users is exactly one. It does simplify a bit the access tracking
logic and saves 4 bytes per panfrost_access_entry at the expense of
additional sparse array derefs, so I'm not sure we want to do that.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>