drm/amdgpu: enable 48-bit IH timestamp counter

By default this timestamp is 32 bit counter. It gets
overflowed in around 10 minutes.

Signed-off-by: Alex Sierra <alex.sierra@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>