drm/amdgpu/display: add missing FP_START/END checks dcn32_clk_mgr.c

Properly handle FP code in dcn32_clk_mgr.c.

Fixes: 265280b9 ("drm/amd/display: add CLKMGR changes for DCN32/321")
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>