drm/radeon/r6xx/r7xx: further safe reg clean up

- remove a few more drm only regs
- remove sampler, alu, bool, loop constant regs.
  They are set via separate packet3's already

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