radeon/vcn: fix poc for hevc encode

radeon/vcn: fix poc for hevc encode

MaxPicOrderCntLsb should be at least 16 according to the spec,
therefore add minimum value check.

Also use poc value passed from st instead of calculation
in slice header encoding.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110673
Cc: mesa-stable@lists.freedesktop.org
V2: Fix typo

V3: Use MAX2 macro instead of coding. Also MaxPicOrderCntLsb
should be power of 2 according to spec.

Signed-off-by: Boyuan Zhang <boyuan.zhang@amd.com>
Acked-by: Leo Liu <leo.liu@amd.com>