gallivm: honor explicit derivatives values for cube maps.

gallivm: honor explicit derivatives values for cube maps.

This is trivial now, though need to make sure we pass all the necessary
derivative values (which is 3 each for ddx/ddy not 2).
Passes piglit arb_shader_texture_lod-texgradcube test.

v2: add the forgotten abs() for all incoming derivatives (discovered
by new piglit arb_shader_texture_lod-texgradcube test, though more by
luck as it was failing only for exactly one pixel...).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>