drm/amd/display: Return max resolution supported by DWB

mode_config's max width x height is 4096x2160 and is higher than DWB's
max resolution 3840x2160 which is returned instead.

Cc: stable@vger.kernel.org
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>