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

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

For a depthstencil buffer with separate stencil,
intel_renderbuffer::region is null. (The regions are kept in hidden depth
and stencil buffers). Since the region is null, intel_map_renderbuffer()
assumed there was no data and returned a null map pointer, which in turn
was dereferenced (!) by MapRenderbuffer's caller.

This patch fixes intel_map_renderbuffer() to map the hidden depth buffer
through the GTT and return that as the mapped pointer. Also, the stencil
bits are scattered and gathered when needed.

Fixes the following Piglit tests on gen7:
    fbo/fbo-readpixels-depth-formats
    hiz/hiz-depth-read-fbo-d24s8
    hiz/hiz-stencil-read-fbo-d24s8
    EXT_packed_depth_stencil/fbo-clear-formats
    EXT_packed_depth_stencil/fbo-depth-GL_DEPTH24_STENCIL8-blit
    EXT_packed_depth_stencil/fbo-depth-GL_DEPTH24_STENCIL8-drawpixels
    EXT_packed_depth_stencil/fbo-depth-GL_DEPTH24_STENCIL8-readpixels
    EXT_packed_depth_stencil/fbo-depthstencil-GL_DEPTH24_STENCIL8-readpixels-24_8
    EXT_packed_depth_stencil/fbo-depthstencil-GL_DEPTH24_STENCIL8-readpixels-FLOAT-and-USHORT
    EXT_packed_depth_stencil/fbo-stencil-GL_DEPTH24_STENCIL8-readpixels

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