drm/fb-helper: Push down modeset lock into FB helpers

Move the modeset locking from drivers into FB helpers.

v2: Also handle intel_connector_add_to_fbdev.

v3: Prevent race in intel_dp_mst with ->detect (Maarten)

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Tested-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com> (v1)
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170704151833.17304-2-daniel.vetter@ffwll.ch