iris: Simplify iris_transfer_map compression logic
When 3915b56e enabled the I915_FORMAT_MOD_Y_TILED_GEN12_RC_CCS
modifier, Clayton found that the lodbias piglit test started taking a long time and timing-out in the CI. I narrowed the issue down to frequent glReadPixels calls and came up with a driver heuristic to detect and work around it. Ultimately however, I decided to fix the issue with piglit!359 (merged) because I couldn't find any other application to benefit from the driver change. This MR contains some code improvements from the driver fix that I think are still useful to keep.
This also addresses part of @jekstrand's comment in #2617 (closed).