drm/etnaviv: Allow DRAW_INSTANCED commands

Vivante GPUs with HALTI0 feature support a DRAW_INSTANCED command in the
command stream to draw a number of instances of the same geometry.

The information that has been figured out about the command can be found
here: https://github.com/etnaviv/etna_viv/blob/master/rnndb/cmdstream.xml#L270



This command is not allowed currently by the DRM driver because it
was not known before. This patch enables parsing it in command
streams and allows using it by userspace drivers.

Signed-off-by: Wladimir J. van der Laan <laanwj@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>