drm/i915: don't read cursor registers on powered down pipes

At i915_display_info, don't call cursor_position() for a disabled
CRTC, since the CRTC may be on a powered down pipe, and this will
cause "Unclaimed register before interrupt" error messages.

Testcase: igt/pm_pc8/debugfs-read
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>