t_dd_dmatmp: Replace fprintf with unreachable

t_dd_dmatmp: Replace fprintf with unreachable

From http://lists.freedesktop.org/archives/mesa-dev/2015-May/084883.html:

    "There are no real error cases here, just dead code.
    validate_render() is supposed to make sure we never call these
    functions if the code can't actually render the primitives. The
    fprintf()+return branches should really just contain assert(0) or
    equivalent."

I also rearranged the if-else-block in render_quad_strip_verts to look
more like the other functions.  A future patch is going to change a
bunch of that code anyway.

v2: Make "unreachable" message more descriptive.  Suggested by Iago.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>