drm: update kerneldoc for changes introduced by commits "16fadc2568e9" and "9671e228fb78"

Describe the new parameter 'bus_flags' to of_get_drm_display_mode() in
the kerneldoc comments and add kerneldoc comments to the new function
drm_bus_flags_from_videomode().

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1471354477-25877-1-git-send-email-LW@KARO-electronics.de