nouveau/dpcd: return EBUSY for aux xfer if the device is asleep

In the Linux kernel, the following vulnerability has been resolved:

nouveau/dpcd: return EBUSY for aux xfer if the device is asleep

If we have runtime suspended, and userspace wants to use /dev/drm_dp_*

then just tell it the device is busy instead of crashing in the GSP

code.

WARNING: CPU: 2 PID: 565741 at drivers/gpu/drm/nouveau/nvkm/subdev/gsp/rm/r535/rpc.c:164 r535_gsp_msgq_wait+0x9a/0xb0 [nouveau]

CPU: 2 UID: 0 PID: 565741 Comm: fwupd Not tainted 6.18.10-200.fc43.x86_64 #1 PREEMPT(lazy)

Hardware name: LENOVO 20QTS0PQ00/20QTS0PQ00, BIOS N2OET65W (1.52 ) 08/05/2024

RIP: 0010:r535_gsp_msgq_wait+0x9a/0xb0 [nouveau]

This is a simple fix to get backported. We should probably engineer a

proper power domain solution to wake up devices and keep them awake

while fw updates are happening.