drm/amd/display: add copy of drm_dp_mst_add_affected_dsc_crtc()

There is a bug in drm_dp_mst_add_affected_dsc_crtc(), that caused mst
dsc hotplugs to not work because atomic check will fail. This was fixed
upstream but is not guaranteed to be in the kernel that dkms is running
on.

Add a copy of drm_dp_mst_add_affected_dsc_crtc() with the fix.
Reference: 2ee394a1 ("drm/dp_mst: Don't return error code when crtc is null")

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>