backend-drm: Drop support to non universal plane drivers

Remove all the backend code to support drivers without universal planes.

From wayland/weston#427

:

"The code needed to support kernels where DRM does not support uiniversal
planes makes the DRM-backend a little more complicated, because it needs
to create fake planes for primary and cursor. The lifetimes of the fake
planes does not match the lifetime of "proper" planes, which is surprising."

Signed-off-by: Igor Matheus Andrade Torrente <igormtorrente@gmail.com>