CHROMIUM: [media] rk3288-vpu: Add helper for encode plane sizes calculation

CHROMIUM: [media] rk3288-vpu: Add helper for encode plane sizes calculation

Currently plane sizes calculation on encoder side is happening only in
vidioc_s_fmt() function, howerver as a prerequisite for further patch
adding further code which needs this operation, this patch adds a common
helper function, which performs this operation.

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

Signed-off-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/289047
Reviewed-by: Pawel Osciak <posciak@chromium.org>
Change-Id: I6e5769667ae40b3fb5758ce9471b4ddd6866183f
Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Signed-off-by: Yakir Yang <ykk@rock-chips.com>