drm/amd/display: Fix compile error from TO_CLK_MGR_INTERNAL

[Why]

On LNX, TO_CLK_MGR_INTERNAL() only works to get clk_mgr_internal from a
given clk_mgr. In clk_mgr_dcn316 struct, the clk_mgr_internal is already
a memeber by the alias 'base'

[How]

Use &clk_mgr->base instead.

Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Leo Li <sunpeng.li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>