vc4: move the draw splitting routine to shared code

vc4: move the draw splitting routine to shared code

This can also be useful for other hardware which has similar limitations
on vertex count per single draw.
The Mali400 has a similar limitation and can reuse this.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/2445>