drm/i915/bdw: Let the memory controller do all the swizzling

Previously, it was possible for the GPU memory accesses to be swizzled
to try to optimize the fetches for tiled buffers. This swizzling was on
top of what the memory controller in the uncore already does.

With broadwell, we drop that GPU side swizzling, and the corresponding
initialization in 3 units (GAM, GT, DE). All those bits are reserved, as
specs put it:

  Before Gen8, there was a historical configuration control field to
  swizzle address bit[6] for in X/Y tiling modes. This was set in three
  different places: TILECTL[1:0], ARB_MODE[5:4], and
  DISP_ARB_CTL[14:13]"

  For Gen8 the swizzle fields are all reserved, and the CPU's memory
  controller performs all address swizzling modifications.

This also means that user space doesn't have to manually swizzle when
accessing tiled buffers from the CPU, and so we always return
I915_BIT_6_SWIZZLE_NONE from i915_gem_detect_bit_6_swizzle(), which
short-circuits the initialization of the registers mentionned above in
i915_gem_init_swizzling().

v2: Refine the explanation a bit more (Daniel)
v3: Make it BDW+ specific (Steve)

Cc: Steve Aarnio <steve.j.aarnio@linux.intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Keep the actual code to set the tiling bits for now, in case
some bios escaped to the wild that uses this - we'd need it for
fastboot.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>