mpeg2: fix construction of reference frames list (SNB, IVB).

mpeg2: fix construction of reference frames list (SNB, IVB).

Fix construction of reference frames list for interlaced streams.
In this case, the array is indexed by frame store ID >> 1 where
bit 0 of frame store ID represents top (0) or bottom (1) field.

Besides, current render target can also be used as a reference
while decoding the second field.

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>
(cherry picked from commit 14f70559b516030d141cce09db54cf49d11df9b2)