drm/i915: only disable enabled planes on intel_fb_restore_mode

We should avoid touching registers that are on the power down well
when we don't need to, because if we touch these registers when the
power well is disabled we'll get tons of "unclaimed register"
messages. This commit fixes some of these messages.

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