drm/tegra: Explicitly set irq_enabled

Since the Tegra DRM driver doesn't use the drm_irq_install() helper, the
irq_enabled flag needs to be set manually in order to make functionality
such as the DRM_IOCTL_WAIT_VBLANK work properly.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>