drm/i915: extract FBC_MULTIPLE_PIPES check

I have two separate refactor ideas that require extracting this to a
separate function. I'm not sure which idea I'll end choosing, but
since both will require extracting this function, let's do this now.

Notice that this is just code moving. Any possible problems with the
current multiple pipes check should be fixed in later commits.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>