drm/mst: Fix possible NULL pointer dereference in drm_dp_mst_process_up_req()

According to DP specification, DP_SINK_EVENT_NOTIFY is also a
broadcast message but as this function only handles
DP_CONNECTION_STATUS_NOTIFY I will only make the static
analyzer that caught this issue happy by not calling
drm_dp_get_mst_branch_device_by_guid() with a NULL guid, causing
drm_dp_mst_process_up_req() to return in the "if (!mstb)" right
bellow.

Fixes: 9408cc94

 ("drm/dp_mst: Handle UP requests asynchronously")
Cc: Lyude Paul <lyude@redhat.com>
Cc: Sean Paul <sean@poorly.run>
Cc: <stable@vger.kernel.org> # v5.5+
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
[added cc to stable]
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200129232448.84704-1-jose.souza@intel.com