drm/radeon: fixes for r6xx/r7xx gfx init

- updated swizzle modes for backend map setup
- fix programming of a few gfx regs
- properly handle pipe/backend setup on LE cards

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