svga: fix clip plane regression after recent tgsi_scan change

svga: fix clip plane regression after recent tgsi_scan change

Before the change "tgsi/scan: use properties for clip/cull distance
writemasks", the tgsi_shader_info::num_written_clipdistance field
was a multiple of four, now it's an accurate count.  In the svga
driver, we need a minor change to the loop test.

Reviewed-by: Charmaine Lee <charmainel@vmware.com>