meson: use a custom target instead of a generator for i965 oa

meson: use a custom target instead of a generator for i965 oa

Generators really are never the thing you want. The problem in this case
is that a generator must create a file that contains any file that the
generated target depends on. Since brw_oa.py doesn't generate such a
file the generated sources are not regenerated even if the xml files
they should depend on changes.

While we could change brw_oa.py to write such a file, that's silly, it
depends on itself and the xml file. So we'll just use a custom target
instead, which will have the correct dependency behavior and doesn't
really add that much code.

Fixes: 3218056e0eb3 ("meson: Build i965 and dri stack")
CC: Ian Romanick <idr@freedesktop.org>
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Acked-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>