drm/amdgpu: Add amdgpu_gfx_off_ctrl function

v2:
   1. drop the special handling for the hw IP
      suggested by hawking and Christian.
   2. refine the variable name suggested by Flora.

This funciton as the entry of gfx off feature.
we arbitrat gfx off feature enable/disable in this
function.

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>