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>

drivers/gpu/drm/amd/amdgpu/amdgpu.h

@@ -950,6 +950,10 @@ struct amdgpu_gfx {
 	/* NGG */
 	struct amdgpu_ngg		ngg;
 
+	/* gfx off */
+	bool                            gfx_off_state; /* true: enabled, false: disabled */
+	struct mutex                    gfx_off_mutex;
+	uint32_t                        gfx_off_req_count; /* default 1, enable gfx off: dec 1, disable gfx off: add 1 */
 	/* pipe reservation */
 	struct mutex			pipe_reserve_mutex;
 	DECLARE_BITMAP			(pipe_reserve_bitmap, AMDGPU_MAX_COMPUTE_QUEUES);
@@ -1774,6 +1778,7 @@ void amdgpu_device_program_register_sequence(struct amdgpu_device *adev,
 					     const u32 array_size);
 
 bool amdgpu_device_is_px(struct drm_device *dev);
+void amdgpu_gfx_off_ctrl(struct amdgpu_device *adev, bool enable);
 /* atpx handler */
 #if defined(CONFIG_VGA_SWITCHEROO)
 void amdgpu_register_atpx_handler(void);