egl/drm: check for dri2_dpy->flush before using the flush extension

egl/drm: check for dri2_dpy->flush before using the flush extension

The current __DRI_DRI2 imples __DRI2_FLUSH. At the same time, one can
use __DRI_IMAGE_DRIVER alongside the latter, so the current check is
confusing at best.

Check for what we use.

v2: Split out from whitespace changes

Reviewed-by: Chad Versace <chadversary@chromium.org> (v1)
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>