drm/i915: s/intel_gen4_compute_page_offset/intel_compute_tile_offset/

Since intel_gen4_compute_page_offset() can now handle tiling formats
all the way down to gen2, rename it to intel_compute_tile_offset().
Not that we actually use it on gen2/3 since there's no DSPSURF etc.
registers which would take a page aligned address.

v2: s/page/tile/ (Daniel)

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/1452625717-9713-7-git-send-email-ville.syrjala@linux.intel.com