anv: Set viewport extents correctly when height is negative

anv: Set viewport extents correctly when height is negative

As per VK_KHR_maintenance1, setting a negative height in the viewport
can be used to get flipped coordinates.  This is, aparently, very useful
when porting D3D apps to Vulkan.  All we need to do to support this is
to make sure we actually set the min and max correctly.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>