drm/prime: Forward declare struct device

We need a declaration of struct device to avoid warnings:

In file included from include/drm/drm_file.h:38:0,
                 from drivers/gpu/drm/drm_file.c:38:
include/drm/drm_prime.h:71:14: warning: 'struct device' declared inside
	  parameter list will not be visible outside of this definition or
	  declaration
          struct device *attach_dev);
		 ^~~~~~

Forward declare it.

Signed-off-by: Laura Abbott <labbott@redhat.com>
Reviewed-by: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1494435925-9457-1-git-send-email-labbott@redhat.com