drm/amd/display: Red screen observed on startup

[Why]
We try to to change new_clocks->dppclk_khz to 100000 when
new_clocks->dppclk_khz is 0

[How]
Don't change new_clocks->dppclk_khz value when new_clocks->dppclk_khz is
0

Signed-off-by: Peikang Zhang <peikang.zhang@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>