drm/amd/debugfs: Expose GFXOFF state to userspace

GFXOFF has two different "state" values: one to define if the GPU is
allowed/disallowed to enter GFXOFF, usually called state; and another
one to define if currently GFXOFF is being used, usually called status.
Even when GFXOFF is allowed, GPU firmware can decide to not used it
accordingly to the GPU load.

Userspace can allow/disallow GPUs to enter into GFXOFF via debugfs. The
kernel maintains a counter of requests for GFXOFF (gfx_off_req_count)
that should be decreased to allow GFXOFF and increased to disallow.

The issue with this interface is that userspace can't be sure if GFXOFF
is currently allowed. Even by checking amdgpu_gfxoff file, one might get
an ambiguous 2, that means that GPU is currently out of GFXOFF, but that
can be either because it's currently disallowed or because it's allowed
but given the current GPU load it's enabled. Then, userspace needs to
rely on the fact that GFXOFF is enabled by default on boot and to track
this information.

To make userspace life easier and GFXOFF more reliable, return the
current state of GFXOFF to userspace when reading amdgpu_gfxoff with the
same semantics of writing: 0 means not allowed, not 0 means allowed.

Expose the current status of GFXOFF through a new file,
amdgpu_gfxoff_status.

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

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