drm/ttm/ttm_bo: Provide a missing 'bulk' description and correct misnaming of 'placement'

'bulk' description taken from another in the same file.

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

 drivers/gpu/drm/ttm/ttm_bo.c:98: warning: Function parameter or member 'bulk' not described in 'ttm_bo_set_bulk_move'
 drivers/gpu/drm/ttm/ttm_bo.c:768: warning: Function parameter or member 'placement' not described in 'ttm_bo_mem_space'
 drivers/gpu/drm/ttm/ttm_bo.c:768: warning: Excess function parameter 'proposed_placement' description in 'ttm_bo_mem_space'

Signed-off-by: Lee Jones <lee@kernel.org>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230317081718.2650744-6-lee@kernel.org