drm/radeon/kms/atom: atom parser fixes

Only reset the reg block on the initial execute
table call; nested calls require the reg block not be
reset on each call.  Also reset the fb window and
io mode.  This matches the upstream parser behavior.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>