drm/i915: Don't try to calculate relative data rates during hw readout

We don't actually read out full plane state during driver startup (only
whether the primary plane is enabled/disabled), so all of the src/dest
rectangles are invalid at this point.  However this calculation was
needless anyway since we re-calculate them from scratch on the very
first atomic transaction after boot anyway.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Kumar Mahesh <mahesh1.kumar@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463439121-28974-2-git-send-email-matthew.d.roper@intel.com