drm/i915: only run hsw_power_well_post_enable when really needed

Only run it after we actually enable the power well. When we're
booting the machine there are cases where we run
hsw_power_well_post_enable without really needing, and even though
this is not causing any real bugs, it is unneeded and causes confusion
to people debugging interrupts.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>