anv: Count image param entries rather than images

anv: Count image param entries rather than images

This is what we're actually storing in the descriptor set and consuming
when we bind surface states.  This commit renames image_count to
image_param_count a few places and moves the decision to not count image
params on gen9+ into anv_descriptor_set.c when we build the layout.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>