i965: fix bogus swizzle in brw_cubemap_normalize

When used with a cube array in VS, failed assertion in ir_validate:

   Assignment count of LHS write mask channels enabled not
   matching RHS vector size (3 LHS, 4 RHS).

To fix this, swizzle the RHS correctly for the writemask.

This showed up in the ARB_texture_gather tests, which exercise cube
arrays in the VS.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Cc: "9.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit 0d7fc10bcd2efb2f96d684b3273b4e0c0b0afef0)