i965: Define and initialize image parameter structure.

i965: Define and initialize image parameter structure.

This will be used to pass image meta-data to the shader when we cannot
use typed surface reads and writes.  All entries except surface_idx
and size are otherwise unused and will get eliminated by the uniform
packing pass.  size will be used for bounds checking with some image
formats and will be useful for ARB_shader_image_size too.  surface_idx
is always used.

v2: Add CS support.  Move the image_params array back to
    brw_stage_prog_data.
v3: Improve documentation.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>