drm/i915: Allow late allocation of request for i915_add_request()

Request preallocation was added to i915_add_request() in order to
support the overlay. However, not all users care and can quite happily
ignore the failure to allocate the request as they will simply repeat
the request in the future.

By pushing the allocation down into i915_add_request(), we can then
remove some rather ugly error handling in the callers.

v2: Nullify request->file_priv otherwise we chase a garbage pointer
when retiring requests.

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