drm/amdgpu: Initialize pipe priority order on graphic initialization

Initialized PIPE_ORDER_TS0/1/2/3 field of SPI_ARB_PRIORITY register to 2.
This set the pipe priority order to:
02 - HP3D, CS_H, GFX, CS_M, CS_L

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>