drm/i915: vlv: W/a for hotplug/manual VGA detection

VGA detection requires the reference clock to be on, so make sure this
is the case.

This fixes VGA hotplug/manual detection where all pipes are off and so
we would normally disable all clocks.

v2:
- Instead of disabling PSR clock gating, force the reference clock on
  through the DPLL_A register. (Kin Chan S <kin.s.chan@intel.com>)

v3:
- Move enabling of the clock to intel_reset_dpio() and use the DPLL_B
  register instead, where we already have a similar tweak for the CRI
  clock. (Ville)

Reported-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>