drm/ttm: cleanup io_mem interface with nouveau

Nouveau is the only user of this functionality and evicting io space
on -EAGAIN is really a misuse of the return code.

Instead switch to using -ENOSPC here which makes much more sense and
simplifies the code.

This could unbreak something as we now cleanly return EAGAIN, but the
chance for this are rather low.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/378237/