drm/radeon/kms/atom: make sure tables are valid (v2)

Check that atom cmd and data tables are valid
before using them.

(v2)
- fix some whitespace errors noticed by Rafał Miłecki
- check a few more cases

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