drm/i915: remove redundant likely/unlikely annotation

unlikely has already included in IS_ERR(), so just
remove redundant likely/unlikely annotation.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190221020819.21832-1-cgxu519@gmx.com