intel: Replace intel_texture_image::stencil_irb with intel_mipmap_tree::stencil_mt...

intel: Replace intel_texture_image::stencil_irb with intel_mipmap_tree::stencil_mt [v3]

For depthstencil textures using separate stencil, we embedded a stencil
buffer in intel_texture_image. The intention was that the embedded stencil
buffer would be the golden copy of the texture's stencil bits. When
necessary, we scattered/gathered the stencil bits between the texture
miptree and the embedded stencil buffer.

This approach had a serious deficiency for mipmapped or multi-layer
textures. Any given moment the embedded stencil buffer was consistent with
exactly one miptree slice, the most recent one to be scattered. This
permitted tests of type A to pass, but broke tests of type B.

Test A:
    1. Create a depthstencil texture.
    2. Upload data into (level=x1,layer=y1).
    3. Read and test stencil data at (level=x1, layer=y1).
    4. Upload data into (level=x2,layer=y2).
    5. Read and test stencil data at (level=x2, layer=y2).

Test B:
    1. Create a depthstencil texture.
    2. Upload data into (level=x1,layer=y1).
    3. Upload data into (level=x2,layer=y2).
    4. Read and test stencil data at (level=x1, layer=y1).
    5. Read and test stencil data at (level=x2, layer=y2).

v2:
   Only allocate stencil miptree if intel->must_use_separate_stencil,
   because we don't make the conversion from must_use_separate_stencil to
   has_separate_stencil until commit
        intel: Use separate stencil whenever possible

v3:
   Don't call ChooseNewTexture in intel_renderbuffer_wrap_miptree() in
   order to determine the renderbuffer format. Instead, pass the format as
   a param to that function.

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