drm/i915: Redo intel_tile_height() as intel_tile_size() / intel_tile_width()

I find more usual to think about tile widths than heights, so changing
the intel_tile_height() to calculate the tile height as
tile_size/tile_width is easier than the opposite to the poor brain.

v2: Reorder arguments for consistency
    Constify dev_priv arguments

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/1452625717-9713-4-git-send-email-ville.syrjala@linux.intel.com