drm/amdgpu: Unset baco dummy mode on nbio v7.9

BACO dummy mode could be set under reset conditions and that affects
framebuffer access. Check If baco dummy mode is set, unset it if so.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Asad Kamal <asad.kamal@amd.com>
Tested-by: Asad Kamal <asad.kamal@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>