drm/radeon/kms/atom: use get_unaligned_le32() for ctx->ps

Noticed on a DEC Alpha.

Start up into console mode caused 15 unaligned accesses, and starting X
caused another 48.

Signed-off-by: Matt Turner <mattst88@gmail.com>
CC: Jerome Glisse <jglisse@redhat.com>
CC: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>