drm: tegra: Check return value of drm_vblank_get()

drm_vblank_get() can return an error, which we should propagate.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
drm: tegra: Stop using drm_vblank_pre/post_modeset()

drm_vblank_pre/post_modeset()'s usage is discourage in their very own
documentation, which recommends using drm_vblank_off/on() instead. For
Tegra this has become a necessity since commit 7ffd7a68511c slightly
changed the semantics of the latter functions and prevents them from
mixing safely with their deprecated predecessors. This resulted in calls
to drm_vblank_get() to fail in the Tegra driver.

This patch ensures that the Tegra driver now only uses
drm_vblank_off/on(), restoring a proper vblank behavior.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>