drm/i915: extract intel_fbc_find_crtc()

I want to make this code a little more complicated, so let's extract
the function first.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>