etnaviv: Use only DRAW_INSTANCED on GC3000+

etnaviv: Use only DRAW_INSTANCED on GC3000+

The blob does this, as DRAW_INSTANCED can replace fully all the other
draw commands. It is also required to handle integer vertex formats.
The other path is only there for compatibility and might go away (or at
least rot to become buggy due to dis-use) in newer hardware.

As a by-effect this changes the behavior for GC3000-, by no longer using
the index offset for DRAW_INDEXED but instead adding it to INDEX_ADDR.
This should make no difference.

Preparation for GC7000 support.

Signed-off-by: Wladimir J. van der Laan <laanwj@gmail.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>