drm/amdgpu: Add mode2 reset support for aldebaran

v1: Aldebaran uses reset control to support mode2 reset. The sequences to
reset and restore hardware context are specific to a particular
configuration.

v2: Clear bus mastering before reset.
Fix coding style issues, drop unwanted variables and info log.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>