drm/i915: protect HSW power well check with IS_HASWELL in redisable_vga

This may need work if other platforms do the same thing, but in the
meantime we should avoid looking at HSW specific bits in this generic
function.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[added IS_BROADWELL too as that needs the same handling (Imre)]
Signed-off-by: Imre Deak <imre.deak@intel.com>
[danvet: Add Imre's missing sob.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>