drm/i915: Try harder to disable trickle feed on VLV

The specs are a bit unclear whether the per-plane trickle feed disable
control exists on VLV. There is another trickle feed disable control
in the MI_ARB register.

After some experimentation it turns out both the DSPCNTR trickle feed
bits and the MI_ARB bit can be toggled. However the DSPCNTR bits don't
seem to have any effect.

The MI_ARB bit, on the other hand, has a noticable effect. I performed
an experiment where I reduced the FIFO size via DSPARB and observed the
effect of the MI_ARB trickle feed bit on the display.

Using a 1920x1080-60 mode, with MI_ARB=0x4 the display started to have
problems with DSPARB=0x42424242, whereas with MI_ARB=0x0 the problems
didn't start until DSPARB=0x09090909. This seems to confirm that the
MI_ARB trickle feed bit actually does work.

So replace the use of the DSPCNTR trickle feed bits with MI_ARB
on VLV.

v2: Amend commit message with results from experimentation

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>