i965/gen7: Enable fragment shader dispatch if the program has image uniforms.

i965/gen7: Enable fragment shader dispatch if the program has image uniforms.

Shaders with image uniforms may have side effects.  Make sure that
fragment shader threads are dispatched if the shader has any image
uniforms.

v2: Use brw_stage_prog_data::nr_image_params to find out if the shader
    has image uniforms instead of checking core mesa data structures
    (Ken).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>