i965: fix textureGrad for cubemaps

i965: fix textureGrad for cubemaps

Fixes bugs exposed by commit
2b1cdb0eddb73f62e4848d4b64840067f1f70865 in:
   ES3-CTS.gtf.GL3Tests.shadow.shadow_execution_frag

No regressions observed in deqp, CTS or Piglit.

v2: address review feedback from Iago Toral:
   - move rho calculation to else branch
   - optimize dx and dy calculation
   - fix documentation inconsistensies

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Signed-off-by: Kevin Rogovin <kevin.rogovin@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91114
Cc: "10.6 11.0" <mesa-stable@lists.freedesktop.org>