mesa-gl: Use swrast gallium driver

Fixes:
../mesa-21.0.0/meson.build:21:0: ERROR: Options "swrast" are not in allowed choices: "auto, i915, i965, r100, r200, nouveau"

with any driver enabled in DRIDRIVERS, do_configure fails with:
../mesa-21.0.0/meson.build:519:4: ERROR: Problem encountered: building dri drivers require at least one windowing system

even after enabling gallium and wayland PACKAGECONFIGs, move DRIDRIVERS_append* from
mesa.inc to mesa recipe.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>