drm/i915: Extract CFB threshold calculation

Right now, there is no threshold (0 means fail, 1 means 1:1 compression
limit). This is to split the function/non-functional change of the next
patch.

The next patch will start to attempt to reduce the amount of CFB space
we need for dire situations. It will be contained within this function.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>