r600g: fix calculation for gpr allocation

r600g: fix calculation for gpr allocation

I've been chasing a geom shader hang on rv635 since I wrote
r600 geom code, and finally I hacked some values from fglrx
in and I could run texelfetch without failures.

This is totally my fault as well, maths fail 101.

This makes geom shaders on r600 not fail heavily.

Cc: "10.6" "11.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>