drm/i915: Change i915_request power well handling

Reorganize the internal i915_request power well handling to use the
reference count just like everyone else. This way all we need to do is
check the reference count and we know whether the power well needs to be
enabled of disabled.

v2: Split he intel_display_power_{get,put} change to another patch.
    Add intel_resume_power_well() to make sure we enable the power
    well on resume

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>