drm: add helper functions to retrieve old and new crtc

drm: add helper functions to retrieve old and new crtc

Add new helper functions, drm_atomic_get_old_crtc_for_encoder
and drm_atomic_get_new_crtc_for_encoder to retrieve the
corresponding crtc for the encoder.

Signed-off-by: Sankeerth Billakanti <quic_sbillaka@quicinc.com>
Signed-off-by: Vinod Polimera <quic_vpolimer@quicinc.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Patchwork: https://patchwork.freedesktop.org/patch/524718/
Link: https://lore.kernel.org/r/1677774797-31063-2-git-send-email-quic_vpolimer@quicinc.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>