i965: Fix textureGrad with cube samplers

i965: Fix textureGrad with cube samplers

We can't use sampler messages with gradient information (like
sample_g or sample_d) to deal with this scenario because according
to the PRM:

"The r coordinate and its gradients are required only for surface
types that use the third coordinate. Usage of this message type on
cube surfaces assumes that the u, v, and gradients have already been
transformed onto the appropriate face, but still in [-1,+1] range.
The r coordinate contains the faceid, and the r gradients are ignored
by hardware."

Instead, we should lower this to compute the LOD manually based on the
gradients and use a different sample message that takes the computed
LOD instead of the gradients. This is already being done in
brw_lower_texture_gradients.cpp, but it is restricted to shadow
samplers only, although there is a comment stating that we should
probably do this also for samplerCube and samplerCubeArray.

Because of this, both dEQP and Piglit test cases for textureGrad with
cube maps currently fail.

This patch does two things:
1) Activates the texturegrad lowering pass for all cube samplers.
2) Corrects the computation of the LOD value for cube samplers.

I had to do 2) because for cube maps the calculations implemented
in the lowering pass always compute a value of rho that is twice
the value we want (so we get a LOD value one unit larger than we
want). This only happens for cube map samplers (all kinds). I am
not sure about why we need to do this, but I suspect that it is
related to the fact that cube map coordinates, when transported
to a specific face in the cube, are in the range [-1, 1] instead of
[0, 1] so we probably need to divide the derivatives by 2 when
we compute the LOD. Doing that would produce the same result as
dividing the final rho computation by 2 (or removing a unit
from the computed LOD, which is what we are doing here).

Fixes the following piglit tests:
bin/tex-miplevel-selection textureGrad Cube -auto -fbo
bin/tex-miplevel-selection textureGrad CubeArray -auto -fbo
bin/tex-miplevel-selection textureGrad CubeShadow -auto -fbo

Fixes 10 dEQP tests in the following category:
dEQP-GLES3.functional.shaders.texture_functions.texturegrad.*cube*

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>