drm/dp_mst: Drop DRM_ERROR() on kzalloc() fail in drm_dp_mst_handle_up_req()

Checkpatch was complaining about this - there's no need for us to print
errors when kzalloc() fails, as kzalloc() will already WARN for us. So,
let's fix that before converting things to make checkpatch happy.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Robert Foss <robert.foss@linaro.org>
Reviewed-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210326203807.105754-20-lyude@redhat.com