Documentation/gpu: Add GFXOFF section

Add a GFXOFF section at "GPU Power Controls" file, explaining what it is
and how userspace can interact with it.

v2: minor tweaks to the documenation (Alex)

Signed-off-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>