drm/vmwgfx/ttm_object: Demote half-assed headers and fix-up another

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/vmwgfx/ttm_object.c:60: error: Cannot parse struct or union!
 drivers/gpu/drm/vmwgfx/ttm_object.c:97: warning: Function parameter or member 'mem_glob' not described in 'ttm_object_device'
 drivers/gpu/drm/vmwgfx/ttm_object.c:97: warning: Function parameter or member 'ops' not described in 'ttm_object_device'
 drivers/gpu/drm/vmwgfx/ttm_object.c:97: warning: Function parameter or member 'dmabuf_release' not described in 'ttm_object_device'
 drivers/gpu/drm/vmwgfx/ttm_object.c:97: warning: Function parameter or member 'dma_buf_size' not described in 'ttm_object_device'
 drivers/gpu/drm/vmwgfx/ttm_object.c:97: warning: Function parameter or member 'idr' not described in 'ttm_object_device'
 drivers/gpu/drm/vmwgfx/ttm_object.c:128: warning: Function parameter or member 'rcu_head' not described in 'ttm_ref_object'
 drivers/gpu/drm/vmwgfx/ttm_object.c:128: warning: Function parameter or member 'tfile' not described in 'ttm_ref_object'
 drivers/gpu/drm/vmwgfx/ttm_object.c:582: warning: Function parameter or member 'dmabuf' not described in 'get_dma_buf_unless_doomed'
 drivers/gpu/drm/vmwgfx/ttm_object.c:582: warning: Excess function parameter 'dma_buf' description in 'get_dma_buf_unless_doomed'

Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Roland Scheidegger <sroland@vmware.com>
Cc: Zack Rusin <zackr@vmware.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Rob Clark <rob.clark@linaro.org>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Zack Rusin <zackr@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210115181601.3432599-10-lee.jones@linaro.org