drm/i915: don't reallocate the compressed FB at every frame

With the current code we just reallocate the compressed FB at every
FBC update: we have X in one frame, then in the other frame we need X
again, but we check "needed < have" instead of "needed <= have".

v2: Rebase after Jani addressed the other problems described in v1.

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>