r600g: add doubles support for CAYMAN

r600g: add doubles support for CAYMAN

Only a subset of AMD GPUs supported by r600g support doubles,
CAYMAN and CYPRESS are probably all we'll try and support, however
I don't have a CYPRESS so ignore that for now.

This disables SB support for doubles, as we think we need to
make the scheduler smarter to introduce delay slots.

[airlied: pushing this to avoid pain of rebasing, it mostly
works on cayman only so far, Glenn has some ideas about
delay slot issues we need to look into. turned off by
default for now]

Signed-off-by: Dave Airlie <airlied@redhat.com>