drm: drop error code for drm_helper_resume_force_mode

No driver cares, and it should generally work. Add a big comment
when drivers can't use this for recompense.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --cc drivers/gpu/drm/drm_crtc_helper.c
Simple merge

diff --cc include/drm/drm_crtc_helper.h
Simple merge