main/mesa: build gallium i915 instead of classic one
Gallium driver used to be less mature and stable, but it's approaching feature parity with classic one. Also it's more performant and fixes some issues. For example, on classic i915, EGL extension required to run modern sway/wlroots (EGL_EXT_image_dma_buf_import) is not present, however it's introduced in gallium i915.