drm/i915/display: Wait at least 2 frames before selective update

BSpec states that the minimum number of frames before selective update
is 2, so making sure this minimum limit is fulfilled.

BSpec: 50422
Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210914212507.177511-2-jose.souza@intel.com