drm/nouveau/device/tegra: fix uninitialized IRQ number

nvkm_device_tegra_new initializes the irq member of the Tegra device
to -1 in order to signal that it is uninitialized. However,
nvkm_device_tegra_fini tests it against 0 to check whether an IRQ has
been allocated or not. This leads to free_irq being called on -1 during
device initialization.

Fix this by using 0 as the uninitialized value everywhere.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>