drm/tegra: dc: Consistently use the same pipe

The hardware pipe numbers don't always match the DRM CRTC indices. This
can happen for example if the first display controller defers probe,
causing it to be registered with DRM after the second display
controller. When that happens the hardware pipe numbers and DRM CRTC
indices become different. Make sure that the CRTC index is always used
when accessing per-CRTC VBLANK data. This can be ensured by using the
drm_crtc_vblank_*() API, which will do the right thing automatically
given a struct drm_crtc *.

Signed-off-by: Thierry Reding <treding@nvidia.com>