drm: fix missing doc v2

flags is new param for drm_syncob_find_fence, so need update doc, reproduce: make htmldocs

reported:
htmldocs: drivers/gpu/drm/drm_syncobj.c:230: warning: Function parameter or member 'flags' not described in 'drm_syncobj_find_fence'

v2: rebased

Signed-off-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181017070318.15483-1-david1.zhou@amd.com