drm/i915/gem: Drop relocation slowpath

Since the relocations are no longer performed under a global
struct_mutex, or any other lock, that is also held by pagefault handlers,
we can relax and allow our fast path to take a fault. As we no longer
need to abort the fast path for lock avoidance, we no longer need the
slow path handling at all.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200311160310.26711-1-chris@chris-wilson.co.uk