egl/x11: advertise __DRI_USE_INVALIDATE for DRI2

egl/x11: advertise __DRI_USE_INVALIDATE for DRI2

Back in 2012 (commit 1e7776ca2bc - egl: Remove bogus invalidate code.)
the loader use of invalidate() was purged as "bogus". One of the factors
defining that statement was the lack of the loader-side invalidate
extension - __DRI_USE_INVALIDATE.

Since then the commit was reverted (commit eed0a80137d - egl: Restore
"bogus" DRI2 invalidate event code.), always performing the driver
invalidate call, although the loader was never updated to expose the
extension.

Do so allowing the driver to do fine grained tuning.

Cc: Eric Anholt <eric@anholt.net>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net