drm: Remove duplicate forward declaration

Forward declarations in C are great but I'm pretty sure one is enough.

Signed-off-by: Dawid Kurek <dawikur@gmail.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170614213518.GA3554@gmail.com