drm/amd/display: fix dcn-specific clk_mgr init_clocks

[Why]
underflow seen on certain monitor setups caused by making dcnxx_init_hw
generic

[How]
by moving dcn20_init_hw into dcn10, we added a dcn-specific clk_mgr
init (dc->clk_mgr->funcs->init_clocks()). Thus, put old clk_mgr
memset in an else statement so both memsets don't get set

Signed-off-by: Martin Leung <martin.leung@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>