drm/ttm: replace context flags with bools v2

The ttm_operation_ctx structure has a mixture of flags and bools. Drop the
flags and replace them with bools as well.

v2: fix typos, improve comments

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/398686/