hwc: Fix Idle fall back when external connected.

Reset idle fall back at the end of prepare, so that irrespective of
whether primary or external is configured first, both will fall back.

Related-to-bug: 8491180
Change-Id: I6206f7cf0cccf71d1b33153044546515029f4e08
Signed-off-by: Iliyan Malchev <malchev@google.com>

Conflicts:
	libhwcomposer/hwc_mdpcomp.cpp