FROMLIST: drm/rockchip: vop: export line flag function

FROMLIST: drm/rockchip: vop: export line flag function

VOP have integrated a hardware counter which indicate the exact display
line that vop is scanning. And if we're interested in a specific line,
we can set the line number to vop line_flag register, and then vop would
generate a line_flag interrupt for it.

For example eDP PSR function is interested in the vertical blanking
period, then driver could set the line number to zero.

This patch have exported a symbol that allow other driver to listen the
line flag event with given timeout limit:
- rockchip_drm_wait_line_flag()

BUG=chrome-os-partner:54785
TEST=Success to call rockchip_drm_wait_line_flag()

Change-Id: Id7605bada87c8ead0e549d4ff113ee49d29ad126
Signed-off-by: Yakir Yang <ykk@rock-chips.com>
(am from https://patchwork.kernel.org/patch/9231675/)
Signed-off-by: Mark Yao <mark.yao@rock-chips.com>
Reviewed-on: https://chromium-review.googlesource.com/349084
Commit-Ready: Sean Paul <seanpaul@google.com>
Tested-by: Sean Paul <seanpaul@google.com>
Reviewed-by: Sean Paul <seanpaul@google.com>