iris: Use isl_surf_get_image_surf instead of hand-rolling it

iris: Use isl_surf_get_image_surf instead of hand-rolling it

For the 3D RT read case on BDW, we can use the ISL helper.  For the 1D
array case, we don't actually need to flatten; we can just smash it to a
2D array surface because the layout for 1D and 2D is the same on BDW.

Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11235>