mesa: Fix glFramebufferTexture*() for depth and stencil attachments

mesa: Fix glFramebufferTexture*() for depth and stencil attachments

This patch solves three bugs.

1. When a texture was attached to the GL_DEPTH_STENCIL_ATTACHMENT point,
Mesa attached the texture only to the depth attachment point
    gl_framebuffer::Attachment[BUFFER_DEPTH]
and failed to attach it to the stencil attachment point
    gl_framebuffer::Attachment[BUFFER_STENCIL]

2. When a texture was attached to the GL_DEPTH_ATTACHMENT point and then
later attached to the GL_STENCIL_ATTACHMENT point, Mesa created two
separate renderbuffer wrappers. This caused a GL error in
glGetFramebufferAttachmentParameteriv().

3. Same as 2, but with depth and stencil juxtaposed.

Fixes Piglit test ARB_framebuffer_object/same-attachment-glFramebufferTexture2D-GL_DEPTH_STENCIL

Note: This is a candidate for the stable branches.
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>