drm/msm/registers: Sync gen_header.py from mesa

Sync from mesa commit 04e2140d8be7 ("freedreno/registers: remove python
3.9 dependency for compiling msm").

Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/673556/

Author: Rob Clark