drm/i915: check the power well on i915_pipe_enabled

This fixes "unclaimed register" messages when the power well is
disabled and there's a GPU hang.

v2: Use the new intel_display_power_enabled().
v3: Use the new domains for intel_display_power_enabled().

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