intel: Kill intel_mipmap_level::nr_images [v4]

intel: Kill intel_mipmap_level::nr_images [v4]

For all texture targets except GL_TEXTURE_CUBE_MAP, the 'nr_images' and
'depth' fields of intel_mipmap_level were identical.  In the exceptional
case, nr_images == 6 and depth == 1.

It is simple to determine if a texture is a cube or not, so the presence
of two fields here was not helpful. Worse, it was confusing. When we
eventually implement GL_ARB_texture_cube_map_array, this mess would have
become even more confusing.

This patch removes 'nr_images' and assigns to 'depth' a consistent
meaning: depth is the number of 2D slices at each miplevel.  The exact
semantics of depth varies according to the texture target:
   - For GL_TEXTURE_CUBE_MAP, depth is 6.
   - For GL_TEXTURE_2D_ARRAY, depth is the number of array slices. It is
     identical for all miplevels in the texture.
   - For GL_TEXTURE_3D, it is the texture's depth at each miplevel. Its
     value, like width and height, varies with miplevel.
   - For other texture types, depth is 1.

As a consequence, parameters were removed from the following function
signatures:
    intel_miptree_set_level_info
        Remove 'nr_images'.

    i945_miptree_layout
    brw_miptree_layout_texture
    brw_miptree_layout_texture_array
        Remove 'slices'.

v2:
   - Replace "It's" with "Its".
   - Remove all hunks in intel_fbo.c. The hunks were spurious and sneaked
     in during a rebase.
   - Remove unneeded hunk in intel_tex_map_image_for_swrast(). It was
     a little refactor of the for-loop's upper bound.

v4:
   In intel_miptree_get_image_offset(), document the conditions under
   which different if-branches are taken.

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