r600g: truncate point sampled texture coordinates

r600g: truncate point sampled texture coordinates

By default the hardware rounds texcoords.  However,
for point sampled textures, the expected behavior is
to truncate.  When we have point sampled textures,
set the truncate bit in the sampler.

Should fix:
https://bugs.freedesktop.org/show_bug.cgi?id=25871

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>