Fix a race condition in nvdebug_{readl,readq,writel,writeq}HEADmaster

When the GPU is powered off, attempts to read any of its registers
(such as via nvdebug_readl()) result in a fatal interrupt. The
pm_runtime_get() call included in nvdebug sent a request to nvgpu
to turn the GPU back on. **However,** this call did not wait for
the power-on command to take effect. This resulted in a race between
nvdebug and the power management logic, meaning that the GPU may not
have powered-on by the time that nvdebug attempted to read its
registers.

Use pm_runtime_get_sync() instead, which explicitly waits for the
power-on command to complete (or fail) before returning. This
eliminates the race condition.

Thank you to Diego Alejandro Parra Guzman
<diego.guzman@tttech-auto.com>, who brought this issue to my
attention.

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