drm/nouveau/gpio: fail if gpu external power is missing

drm/nouveau/gpio: fail if gpu external power is missing

Currently, nouveau doesn't check if GPU is missing power. This
patch makes nouveau fail when this happens on latest GPUs.

It checks GPIO function 121 (External Power Emergency), which
should detect power problems on GPU initialization.

This can be disabled with nouveau.config=NvPowerChecks=1

Tested on TU104, GP106 and GF100.

v3:
*  Add config override for disabling power checks

Signed-off-by: Mark Menzynski <mmenzyns@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>