drm/amd/display: dc/dce60: set max_cursor_size to 64

[Why]
Issue in the Mouse cursor size in Linux Desktop Environments

[How]
In DCE6 dc->caps.max_cursor_size need to be set as 64 instead of 128

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mauro Rossi <issor.oruam@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>