drm/ttm/ttm_device: Demote kernel-doc abuses

drm/ttm/ttm_device: Demote kernel-doc abuses

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

 drivers/gpu/drm/ttm/ttm_device.c:42: warning: Function parameter or member 'ttm_global_mutex' not described in 'DEFINE_MUTEX'
 drivers/gpu/drm/ttm/ttm_device.c:42: warning: expecting prototype for ttm_global_mutex(). Prototype was for DEFINE_MUTEX() instead
 drivers/gpu/drm/ttm/ttm_device.c:112: warning: Function parameter or member 'ctx' not described in 'ttm_global_swapout'
 drivers/gpu/drm/ttm/ttm_device.c:112: warning: Function parameter or member 'gfp_flags' not described in 'ttm_global_swapout'
 drivers/gpu/drm/ttm/ttm_device.c:112: warning: expecting prototype for A buffer object shrink method that tries to swap out the first(). Prototype was for ttm_global_swapout() instead

Cc: Christian Koenig <christian.koenig@amd.com>
Cc: Huang Rui <ray.huang@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210416143725.2769053-28-lee.jones@linaro.org
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>