i965/miptree: Switch to isl_surf::samples

i965/miptree: Switch to isl_surf::samples

v2 (Jason):
   - Don't trigger miptree re-creation in vain later on with ISL
     based. Core GL uses zero to indicate single sampled while
     ISL uses one - this would cause intel_miptree_match_image()
     to always fail.
   - Now that native miptree is already using sample number of
     one, there is no need for MAX2() when converting to ISL.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>