drm/i915: Change plane_config to store a tiling_mode
Rather than having "tiled" meaning "is it X-tiled?" convert the field to
explicitely store the tiling mode. The code doesn't have to change much
as 1 is conveniently I915_TILING_X.
This is to accommodate future changes around tiling modes and scannout
buffers.
v2: Rebase on top of Ander's "Make intel_crtc->config a pointer"