drm/dp_mst: Don't return error code when crtc is null

[Why]
In certain cases the crtc can be NULL and returning -EINVAL causes
atomic check to fail when it shouln't. This leads to valid
configurations failing because atomic check fails.

[How]
Don't early return if crtc is null

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
[added stable cc]
Signed-off-by: Lyude Paul <lyude@redhat.com>
Fixes: 8ec04671 ("drm/dp_mst: Add helper to trigger modeset on affected DSC MST CRTCs")
Cc: <stable@vger.kernel.org> # v5.6+
Link: https://patchwork.freedesktop.org/patch/msgid/20200814170140.24917-1-Bhawanpreet.Lakha@amd.com