drm/amdgpu/dm: Fix NULL pointer crash during DP MST hotplug

This patch checks the return value of the function
dc_link_add_remote_sink before using it. This was causing
a crash during consecutive hotplugs of DP MST displays.

Cc: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Shashank Sharma <shashank.sharma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>