i965: Zero out {point_sprite,flat}_enables in calculate_attr_overrides.

i965: Zero out {point_sprite,flat}_enables in calculate_attr_overrides.

calculate_attr_overrides is responsible for computing the point sprite
and flat-shading enable bitfields.  It does so by OR'ing in a bunch of
bits.  However, it relied on the caller to set the initial value to
zero.  This is pretty fragile - if the caller neglects to zero out those
variables, then the enable bitfields end up full of garbage, which shows
up as random things being flat-shaded.

This patch moves the zero-initialization into calculate_attr_overrides,
so that the computation is completely in one place.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>