i965/miptree: Add PRM references for most struct members (v2)

i965/miptree: Add PRM references for most struct members (v2)

Add comments that link the driver's miptree structures to the hardware
structures documented in the PRM.  This provides sorely needed
orientation to developers new to the miptree code. And for miptree
veterans, this clarifies some of the more obscure miptree data.

For each driver struct field that closely corresponds to a
hardware struct field, add a PRM reference to that hardware field's
name. For example,

    struct intel_mipmap_tree {
       ...
       /**
        * @brief One of GL_TEXTURE_2D, GL_TEXTURE_2D_ARRAY, etc.
        *
        * @see RENDER_SURFACE_STATE.SurfaceType
        * @see RENDER_SURFACE_STATE.SurfaceArray
        * @see 3DSTATE_DEPTH_BUFFER.SurfaceType
        */
       GLenum target;
       ...
    };

Also annotate the INTEL_MSAA_LAYOUT_* enums with the name of the PRM
sections that documents the layout.

v2: Replace "2D subimage" with "slice", and define what a "slice" is.
    For Ben.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com> (v1)
Reviewed-by: Ben Widawsky <benjamin.widawsky@intel.com> (v1)