drm/amdgpu/display: fix vega12/20 handling in dal_asic_id.h

- Remove unused ASICREV_IS_VEGA12_p() macro
- Fix ASICREV_IS_VEGA12_P() macro to properly check against vega20

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>