drm/prime: Remove duplicate forward declaration

struct device is forward-declared twice. Remove the second instance.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180111201730.30708-1-thierry.reding@gmail.com