drm/amdgpu: Basic emulation support

Add amdgpu_emu_mode module parameter to control the emulation mode
Avoid vbios operation on emulation since there is no vbios post duirng emulation,
use the common hw_init to simulate the post

Signed-off-by: Shaoyun Liu <Shaoyun.Liu@amd.com>
Acked-By: Alex Deucher <alexander.deucher@amd.com>
Acked-By: Christian Konig <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>