drm/tegra: Don't use vblank_disable_immediate on incapable driver.

Tegra would not only need a hardware vblank counter that
increments at leading edge of vblank, but also support
for instantaneous high precision vblank timestamp queries, ie.
a proper implementation of dev->driver->get_vblank_timestamp().

Without these, there can be off-by-one errors during vblank
disable/enable if the scanout is inside vblank at en/disable
time, and additionally clients will never see any useable
vblank timestamps when querying via drmWaitVblank ioctl. This
would negatively affect swap scheduling under X11 and Wayland.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/tegra/drm.c

@@ -173,7 +173,6 @@ static int tegra_drm_load(struct drm_device *drm, unsigned long flags)
 	drm->irq_enabled = true;
 
 	/* syncpoints are used for full 32-bit hardware VBLANK counters */
-	drm->vblank_disable_immediate = true;
 	drm->max_vblank_count = 0xffffffff;
 
 	err = drm_vblank_init(drm, drm->mode_config.num_crtc);