drm/fb-helper: dpms_legacy(): Only set on connectors in use

For each enabled crtc the functions sets dpms on all registered connectors.
Limit this to only doing it once and on the connectors actually in use.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Fixes: 023eb571 ("drm: correctly update connector DPMS status in drm_fb_helper")
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190326175546.18126-3-noralf@tronnes.org