drm/i915: Use fb modifiers for display tiling decisions

Soon the fence tiling mode may not always match the fb modifier
even for X tiled buffers. So let's use the fb modifier
consistently for all display tiling decisions.

v2: Rebased due to s/ring/engine/
v3: Rebased due to s/engine/ring/ O_o
v4: Rebase due to i915_gem_object_get_tiling() & co.

Reviewed-by: Matthew Auld <matthew.auld@intel.com> (v1)
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470821001-25272-6-git-send-email-ville.syrjala@linux.intel.com