drm/amd/display: Remove 300Mhz minimum disp clk limit.

300Mhz disp clk limit was a workaround that was fixed in SMU and is no
longer needed.

Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>