drm/i915: Extract tile_row_size for fencing

Computing the tile row size of a tiled object (for use with fence
registers) is repeated, so extract it to a common helper.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170109161613.11881-1-chris@chris-wilson.co.uk


Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>