drm/i915: print the correct amount of bytes allocated for the CFB

And also print the threshold. I was surprised to see a log message
claiming the CFB size was 32mb when there was less than 24mb available
for it.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>