drm: If available use atomic state in getcrtc ioctl

This way drivers fully converted to atomic don't need to update these
legacy state variables in their modeset code any more.

Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

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