drm/amd/display: Move drm_dp_mst_atomic_check() to the front of dc_validate_global_state()

[Why]
Need to do atomic check first, then validate global state.
If not, when connecting both MST and HDMI displays and
set a bad mode via xrandr, system will hang.

[How]
Move drm_dp_mst_atomic_check() to the front of
dc_validate_global_state().

Signed-off-by: Zhan Liu <zhan.liu@amd.com>
Reviewed-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>