mesa: automake: rework the source generation rules

mesa: automake: rework the source generation rules

Same logic as previous commit applies.

Additionally remove the odd (set -e/mv/INDENT) from the rules.
The last one is the only one we remotely care about, if reading the
generated sources.

Upcoming work from DylanB which will replace the existing python
scripts with ones that produce more readable output anyway.

Cc: 11.0 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>