CHROMIUM: [media] rk3288-vpu: Add helper to get context operating mode

CHROMIUM: [media] rk3288-vpu: Add helper to get context operating mode

Currently there is only one place in the code checking whether given
context is decoding or encoding. However as a prerequisite for further
patch adding more such checks, this patch adds a common helper function
which returns appropriate enum value depending on operating mode of
given context.

BUG=chrome-os-partner:41585
TEST=AppRTC loopback

Signed-off-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/288661
Reviewed-by: Pawel Osciak <posciak@chromium.org>
Change-Id: Ide145770ca77897ada3cb878c4b9c9787824a827
Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Signed-off-by: Yakir Yang <ykk@rock-chips.com>