drm/i915: Remove i915_gem_evict_inactive()

This was only used by one external caller who would just be as happy
with evict-everything, so perform the replacement and make the function
private.

In the process we note that unbinding the inactive list should not fail,
and make it a warning instead.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>