drm/i915: Pull intel_init_power_well() out of intel_modeset_init_hw()

The init and resume codepaths want to handel the power well in slightly
different ways, so pull the power well init out from
intel_modeset_init_hw() which gets called in both cases.

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