drm/i915: s/tile_width/tile_width_bytes/

Make if clear whether we're talking tile widths in bytes or in pixels.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1455569699-27905-3-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>