drm/amd/amdgpu: Add gfxoff debugfs entry

Write a 32-bit value of zero to disable GFXOFF and write a 32-bit
value of non-zero to enable GFXOFF.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>