anv: Fix check for isl_fmt in assert

Checking isl_fmt returned value in assert seems appropriate
instead of format variable.

Fixes: f1654fa7 "anv/android: support creating images from external format"
Signed-off-by: Nataraj Deshpande <nataraj.deshpande@intel.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Sagar Ghuge <sagar.ghuge@intel.com>
(cherry picked from commit d6724471)