st/mesa/glsl/i965: move ImageUnits and ImageAccess fields to gl_program

st/mesa/glsl/i965: move ImageUnits and ImageAccess fields to gl_program

Having it here rather than in gl_linked_shader allows us to simplify
the code.

Also it is error prone to depend on the gl_linked_shader for programs
in current use because a failed linking attempt will free infomation
about the current program. In i965 we could be trying to recompile
a shader variant but may have lost some required fields.

We drop the memset on ImageUnits because gl_program is already
created using rzalloc().

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