drm/i915/fbc: replace a loop with drm_atomic_get_existing_crtc_state()

Much simpler. Thanks to Ville for pointing this.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1478883461-20201-3-git-send-email-paulo.r.zanoni@intel.com