drm/rockchip: dw-mipi-dsi: fix generic packet status check

We want to check that both the GEN_CMD_EMPTY and GEN_PLD_W_EMPTY bits
are set so we can't just check "val & mask" because that will be true if
either bit is set.

Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Chris Zhong <zyw@rock-chips.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170224125506.21533-6-john@metanate.com