drm/amd/display: Don't clear ref_dtbclk value

[Description]
ref_dtbclk value is assigned in clk_mgr_construct,
but the clks struct is cleared in init_clocks.
Make sure to restore the value or we will get
0 value for ref_dtbclk in DCN31.

Reviewed-by: Chris Park <Chris.Park@amd.com>
Acked-by: Jasdeep Dhillon <jdhillon@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>