drm/amdgpu: add vega12 to asic_type enum

Add vega12 to amd_asic_type enum and amdgpu_asic_name[].

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>