drm/amd/display: Stop amdgpu_dm initialize when stream nums greater than 6

[Why]
Coverity reports OVERRUN warning. Should abort amdgpu_dm
initialize.

[How]
Return failure to amdgpu_dm_init.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>