drm/radeon/kms: check for DP MST mode in a few more places (v2)

DP MST is DP multi-stream support, part of DP 1.2.

v2: switch to a helper macro as suggested by Michel.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>