intel: Fix swrast_render_start() for depthstencil buffers with separate stencil

intel: Fix swrast_render_start() for depthstencil buffers with separate stencil

1. Don't map the depthstencil buffer twice

   Place a guard in intel_renderbuffer_map() to prevent a renderbuffer
   from being mapped twice. This happened if a single buffer was attached to
   the framebuffer's depth and stencil attachment points.  (Interestingly,
   because intel_map_renderbuffer_gtt() is idempotent, the double mapping did
   not cause bugs for depthstencil buffers *without* separate stencil).

2. Stop overriding gl_framebuffer::_DepthBuffer,_StencilBuffer

   Normally, if a depthstencil buffer is attached to the framebuffer's
   depth attachment point, then _mesa_update_framebuffer() installs
   a wrapper depth renderbuffer at gl_framebuffer::_DepthBuffer. Ditto for
   the stencil attachment point and gl_framebuffer::_StencilBuffer

   A depthstencil intel_renderbuffer with separate stencil contains hidden
   depth and stencil renderbuffers, which are the *real* renderbuffers. In
   order to force swrast to work, we were installing, in
   brw_update_draw_buffer(), the hidden renderbuffers at
   gl_framebuffer::_DepthBuffer and _StencilBuffer, thus overriding the
   behavior of _mesa_update_framebuffer().  However, now that
   intel_renderbuffer_map() is implemented with MapRenderbuffer(),
   overriding _mesa_update_framebuffer's introduces bugs.  This patch
   removes the override code.

Fixes several Piglit tests on gen7.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>