i965: Bind null render targets for shadow sampling + color.

Portal 2 appears to bind RGBA8888_UNORM textures to a sampler2DShadow,
and calls shadow2D() on it.  This causes undefined behavior in OpenGL.

Unfortunately, our sampler appears to hang in this scenario, which is
not acceptable.  Just give them a null surface instead, which returns
all zeroes.

Fixes GPU hangs in Portal 2 on Kabylake.

Huge thanks to Jason Ekstrand for noticing this crazy behavior while
sifting through crash dumps.

Cc: [email protected]
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104487
Reviewed-by: Topi Pohjolainen <[email protected]>
Reviewed-by: Jason Ekstrand <[email protected]>

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