[Why]
The visual corruption due to low display clock value.
Observed on Carrizo 4K@60Hz.

[How]
There was earlier patch for dce_update_clocks:
Adding +15% workaround also to to dce11_update_clocks
Signed-off-by: Roman Li <Roman.Li@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>