freedreno/registers: split header build into subdirs

Instead of building the adreno/foo.xml headers from the toplevel, split
out a subdir().  This fits better with how meson likes things to be
structured.  But it does require fixing a bit about how gen_header.py
resolves imports, ie. it cannot assume the src file is at the root of
the $RNN_PATH.

This is needed for the next patch, to add support for installing the
register database for use with installed tools.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6154>