[Why]
Carrizo and Stoney have severe corruption when trying to power
4k 60 monitors over HDMI connectors that support 4k 60.

Carrizo and Stoney require retimers and redrivers to support 4k 60
over HDMI.  This driver does not currently support these.  Thus, 4k 60
HDMI (and all other modes requiring over 300MHz) should be disabled.

[How]
Reduce the dce11 HDMI pixel clock cap to 300000kHz.
Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>