drm/i915: Remove the disabling of VHR unit clock gating for HSW

There's is another register (a read only, so no harm done) at 0x42020 on
Haswell GPUs. Let's just remove the write from the copy&paste that
introduced haswell_init_clock_gating().

A note for the interested reader, it does seem we have a duplication of
the 0x42020 register definition, hence the removal of 2 writes. That
duplication could be the object of a later patch.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>