drm: Extract drm_device.h

I need this to untangle an include loop in the next patch.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170802115604.12734-2-daniel.vetter@ffwll.ch