drm/amd/powerplay: centralize all buffer allocation in sw_init phase

To fit common design. And this can simplify the buffer deallocation.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>