mi-builder-test: Update mi_hw dict values gen->gfx
Needed if mesa has commit 66f653597458e930e96210a30e24bed528733d23 ("intel: Rename genx keyword to gfxx in build files").
Without this, tests in CI skip with: "No mi_builder was built/installed for this platform."
Acked-by: Caio Oliveira caio.oliveira@intel.com