etnaviv: fix varying interpolation

etnaviv: fix varying interpolation

It seems that newer cores don't use the PA_ATTRIBUTES to decide if the
varying should bypass the flat shading, but derive this from the component
use. This fixes flat shading on GC880+.

VARYING_COMPONENT_USE_POINTCOORD is a bit of a misnomer now, as it isn't
only used for pointcoords, but missing a better name I left it as-is.

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