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"

Reviewed-By: Tvrtko Ursulin <tvrtko.ursulin@intel.com> (v1)
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>