gpu: host1x: Reserve VBLANK syncpoints at initialization

On T20-T148 chips, the bootloader can set up a boot splash
screen with DC configured to increment syncpoint 26/27
at VBLANK. Because of this we shouldn't allow these syncpoints
to be allocated until DC has been reset and will no longer
increment them in the background.

As such, on these chips, reserve those two syncpoints at
initialization, and only mark them free once the DC
driver has indicated it's safe to do so.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>