i965: Store image_param in brw_context instead of prog_data

i965: Store image_param in brw_context instead of prog_data

This burns an extra 10k of memory or so in the case where you don't have
any images.  However, if you have several shaders which use images, this
should be much less memory.  It also gets rid of a part of prog_data
that really has nothing to do with the compiler.

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