mesa: Make _mesa_is_stencil_format() consistent with _mesa_is_depth_format().

mesa: Make _mesa_is_stencil_format() consistent with _mesa_is_depth_format().

There was only one consumer of this API, meta.c, which was intending
to ask "is this format just stencil index (and nothing else)?".
Instead, if one tried to glDrawPixels of GL_DEPTH_STENCIL-type
formats, it would just try to draw the stencil parts.  Nothing good
came of this.

This function looks rather silly at this point, but I'm leaving it in
place to be the obvious parallel API to _mesa_is_depth_format().  Note
that if you want the old behavior, you should use it as
(_mesa_is_stencil_format() || _mesa_is_depthstencil_format()) like is
commonly done for depth-related tests.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>