drm/msm/dsi: add API to take DSI register snapshot

Add an API to take a snapshot of DSI controller registers. This API
will be used by the msm_disp_snapshot module to capture the DSI
snapshot.

Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/1618606645-19695-4-git-send-email-abhinavk@codeaurora.org


Signed-off-by: Rob Clark <robdclark@chromium.org>