r600g: add support for geom shaders to r600/r700 chipsets (v2)

r600g: add support for geom shaders to r600/r700 chipsets (v2)

This is my first attempt at enabling r600/r700 geometry shaders,
the basic tests pass on both my rv770 and my rv635,

It requires this kernel patch:
http://www.spinics.net/lists/dri-devel/msg52745.html

v2: address Alex comments.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>