anv: Remove default scissor and viewport concepts

anv: Remove default scissor and viewport concepts

Users should never provide a scissor or viewport count of 0 because
they are required to set such state in a graphics pipeline. This
behavior was previously only used in Meta, which actually just
disables those hardware operations at pipeline creation time.

Kristian noticed that the current assignment of viewport count
reduces the number of viewport uploads, so it is not removed.

Signed-off-by: Nanley Chery <nanley.g.chery@intel.com>
Reviewed-by: Kristian Høgsberg Kristensen <kristian.h.kristensen@intel.com>