drm/amd/display: DCN2.x Do not program DPPCLK if same value

[WHY]
Programming DPPCLK to the same value currently set may cause
underflow while playing video in certain conditions.

[HOW]
Only program DPPCLK if clock is not the same as the
previous value programmed.

Signed-off-by: Sung Lee <sung.lee@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>