drm/tegra: dc: Add missing call to drm_vblank_on()

When the CRTC is enabled, make sure the VBLANK machinery is enabled.
Failure to do so will cause drm_vblank_get() to not enable the VBLANK on
the CRTC and VBLANK-synchronized page-flips won't work.

While at it, get rid of the legacy drm_vblank_pre_modeset() and
drm_vblank_post_modeset() calls that are replaced by drm_vblank_on()
and drm_vblank_off().

Reported-by: Alexandre Courbot <acourbot@nvidia.com>
Tested-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>