drm/i915/fbc: use drm_atomic_get_existing_crtc_state when appropriate

Use drm_atomic_get_existing_crtc_state() instead of looping through
the CRTC states and checking if the FBC CRTC is there.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1478883461-20201-6-git-send-email-paulo.r.zanoni@intel.com