drm/amd: Read IMU FW version from scratch register during hw_init

If the IMU version wasn't discovered from the header, such as when
the firmware was directly loaded by PSP then there is no firmware
version to show to userspace from sysfs or IOCTL.

The IMU F/W stores the version in the first scratch register though,
so fetch it in these cases to let the driver export.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>