nvc0: create screen fence objects with coherent attribute

nvc0: create screen fence objects with coherent attribute

This is required on non-coherent architectures to ensure the value of
the fence is correct at all times. Failure to do this results in the
display freezing for a few seconds every now and then on Tegra.

The NOUVEAU_BO_COHERENT is a no-op for coherent architectures, so behavior
on x86 should not be affected by this patch.

Also bump the required libdrm version to 2.4.62, which introduced this
flag.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Martin Peres <martin.peres@free.fr>