Bart,
Surround scsi_execute() calls with scsi_autopm_get_device() and scsi_autopm_put_device(). Note: removing sr_mutex protection from the scsi_cd_get() and scsi_cd_put() calls is safe because the purpose of sr_mutex is to serialize cdrom_*() calls.
This patch avoids that complaints similar to the following appear in the kernel log if runtime power management is enabled:
Applied to 4.18/scsi-fixes, thanks!