i965/shader: Get rid of the shader, prog, and shader_prog fields

i965/shader: Get rid of the shader, prog, and shader_prog fields

Unfortunately, we can't get rid of them entirely.  The FS backend still
needs gl_program for handling TEXTURE_RECTANGLE.  The GS vec4 backend still
needs gl_shader_program for handling transfom feedback.  However, the VS
needs neither and we can substantially reduce the amount they are used.
One day we will be free from their tyranny.

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