i965: isl: Move the MCS gen7+ assertion into ISL

This is useful for every user of ISL. Drop the comment along the way to
match similar functions in ISL.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>

src/intel/isl/isl.c

@@ -1642,6 +1642,8 @@ isl_surf_get_mcs_surf(const struct isl_device *dev,
                       const struct isl_surf *surf,
                       struct isl_surf *mcs_surf)
 {
+   assert(ISL_DEV_GEN(dev) >= 7);
+
    /* It must be multisampled with an array layout */
    assert(surf->samples > 1 && surf->msaa_layout == ISL_MSAA_LAYOUT_ARRAY);