drm/ast: Fixed system hanged if disable P2A

The original ast driver will access some BMC configuration through P2A bridge
that can be disabled since AST2300 and after.
It will cause system hanged if P2A bridge is disabled.
Here is the update to fix it.

Signed-off-by: Y.C. Chen <yc_chen@aspeedtech.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>