drm/i915: Use intel_tile_{size,width,height}() in intel_gen4_compute_page_offset()

Make intel_gen4_compute_page_offset() ready for other tiling formats
besied X-tile by getting the tile dimensions through
intel_tile_{size,width,height}().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1452625717-9713-6-git-send-email-ville.syrjala@linux.intel.com