i965: Lower textureGrad() with samplerCubeShadow.

i965: Lower textureGrad() with samplerCubeShadow.

We weren't lowering textureGrad() with samplerCubeShadow because I
couldn't figure out the LOD calculations.  It turns out they're easy:
you just have to use 1 for the depth.  This causes it to pass
oglconform's four tests.

(cherry picked from commit 613e64060c2bc58c3920c3c6ff77291642ba0e23)

When cherry-picking this to stable, I've reordered this before the
Haswell commit that would have introduced a regression.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Tested-by: Anuj Phogat <anuj.phogat@gmail.com> (original)
Tested-by: Ian Romanick <idr@freedesktop.org> (original)