drm/amd/display: Raise dispclk value for Polaris

[Why]
The visual corruption due to low display clock value observed on some
systems

[How]
There was earlier patch for dspclk:
'drm/amd/display: Raise dispclk value for dce_update_clocks'
Adding +15% workaround also to to dce112_update_clocks

Signed-off-by: Roman Li <Roman.Li@amd.com>
Reviewed-by: Leo Li <Sunpeng.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>