i965: Add a helper function intel_get_tile_dims()

i965: Add a helper function intel_get_tile_dims()

V2:
- Do the tile width/height computations in the new helper
  function and use it later in intel_miptree_get_tile_masks().
- Change the name to intel_get_tile_dims().

V3: Return the tile_h in number of rows in place of bytes.
    Document the units of tile_w, tile_h parameters.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Chad Versace <chad.versace@intel.com>