media: hantro: HEVC: Fix reference frames management

PoC shall be int the range of -2^31 to 2^31 -1
(HEVC spec section 8.3.1 Decoding process for picture order count).
The current way to know if an entry in reference picture array is free
is to test if PoC = UNUSED_REF. Since UNUSED_REF is defined as '-1' that
could lead to decode issue if one PoC also equal '-1'.
PoC with value = '-1' exists in conformance test SLIST_B_Sony_9.

Change the way unused entries are managed in reference pictures array to
avoid using PoC to detect then.

This patch doesn't change fluster HEVC score.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Reviewed-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>