drm/amd/powerplay: add vega20_inc.h (v2)

v2: use thm 11.0.2 headers

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