automake: compact gallium/drivers and gallium/winsys makefiles

Rather than having two separate almost empty and identical makefiles,
compact them thus improving the configure and build time.
Additionally this makes the automake build symmetrical to the scons
and android one.

v2: Rebase on top of vc4, compact drivers + winsys on a single line.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
5 files changed