drm/amd/display: Use max memclk variable when setting max memclk

[Description]
In overclocking scenarios the max memclk could be higher
than the DC mode limit. However, for configs that don't
support MCLK switching we need to set the max memclk to
the overclocked max instead of the DC mode max or we
could result in underflow.

Reviewed-by: Samson Tam <samson.tam@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>