hevcdec: remove HEVCContext usage from hevc_sei

Based on the H264 SEI implementation.

This will be mainly useful once support for SEI messages that can be
used by the hevc parser are implemented, like Picture Timing.

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>