drm/i915: vlv: clear master interrupt flag when disabling interrupts

Not clearing this flag causes spurious interrupts at least in D3 state,
so before enabling RPM we need to fix this. We were already setting this
flag when enabling interrupts, only clearing it was missing.

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>