Return value for mipi_dsi_shutdown_peripheral() is unchecked.
Check it and return any errors if they come up. Even if
mipi_dsi_shutdown_peripheral() fails, continue attempting to
disable.

Cc: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180116222217.240939-1-seanpaul@chromium.org