drm/i915: kill fbc.uncompressed_size

Directly call intel_fbc_calculate_cfb_size() in the only place that
actually needs it, and use the proper check before removing the stolen
node. IMHO, this change makes our code easier to understand.

v2: Use drm_mm_node_allocated() (Chris).

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/