drm/i915: Refactor intel_surf_alignment()

Pull the code to determine the surface alignment for both linear and
tiled surfaces into a separate function intel_surf_alignment(). This
will be used not only for the vma alignment but actually aligning
the plane SURF once SKL+ starts using intel_compute_page_offset()
(since SKL+ needs >4K alignment for tiled surfaces too).

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/1452625717-9713-8-git-send-email-ville.syrjala@linux.intel.com