drm/i915/display: Ensure PSR gets disabled if no encoders in new state

Currently PSR is left enabled when all planes are disabled if there
is no attached encoder in new state. This seems to be causing FIFO
underruns.

Fix this by checking if encoder exists in new crtc state and disable
PSR if it doesn't.

v2: Unify disable logic with existing

Cc: Mika Kahola <mika.kahola@intel.com>
Reported-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220711111750.881552-1-jouni.hogander@intel.com