This adds support to the mesa state tracker for ARB_texture_multisample.

hardware doesn't seem to use a different texture instructions, so
I don't think we need to create one for TGSI at this time.

Thanks to Marek for fixes to sample number picking.

v2: idr pointed out a bug in how we picked the max sample counts,
use new internal format chooser interface to pick proper answers.
v3: use st_choose_format directly, it was okay, fix anding of masks.

Reviewed-by: Marek Olšák <maraeo@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>