drm/amdgpu:Correct max uvd handles

Max uvd handles should use adev->uvd.max_handles instead of
AMDGPU_MAX_UVD_HANDLES here.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org