i965/msaa: Properly handle sliced layout for Gen7.

i965/msaa: Properly handle sliced layout for Gen7.

Starting in Gen7, there are two possible layouts for MSAA surfaces:

- Interleaved, in which additional samples are accommodated by scaling
  up the width and height of the surface.  This is the only layout
  available in Gen6.  On Gen7 it is used for depth and stencil
  surfaces only.

- Sliced, in which the surface is stored as a 2D array, with array
  slice n containing all pixel data for sample n.  On Gen7 this layout
  is used for color surfaces.

The "Sliced" layout has an additional requirement: it must be used in
ARYSPC_LOD0 mode, which means that the surface doesn't leave any extra
room between array slices for miplevels other than 0.

This patch modifies the surface allocation functions to use the
correct layout when allocating MSAA surfaces in Gen7, and to set the
array offsets properly when using ARYSPC_LOD0 mode.  It also modifies
the code that populates SURFACE_STATE structures to ensure that
ARYSPC_LOD0 mode is selected in the appropriate circumstances.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>