drm/amdgpu: Fix max_vblank_count value for current display engines

The value was much too low, which could cause the userspace visible
vblank counter to move backwards when the hardware counter wrapped
around.

Ported from radeon commit:
b0b9bb4d



Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org