drm/i915: Extract a intel_power_well_enable() function

We need a bit book keeping around power wells' ops->enable(), namely a
nice debug message and updating hw_enabled. Let's introduce a
intel_power_well_enable() function to make sure all the callers do the
same things.

v2 (from Paulo):
  - s/i915_power_well_enable/intel_power_well_enable/ since everything
    else on this file uses intel_ instead of i915_.
  - Fix typo in commit message.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>