drm/bridge: tc358768: Fix DSI command tx

drm/bridge: tc358768: Fix DSI command tx

commit 32c4514455b2b8fde506f8c0962f15c7e4c26f1d upstream.

Wait for the command transmission to be completed in the DSI transfer
function polling for the dc_start bit to go back to idle state after the
transmission is started.

This is documented in the datasheet and failures to do so lead to
commands corruption.

Fixes: ff1ca6397b1d ("drm/bridge: Add tc358768 driver")
Cc: stable@vger.kernel.org
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20240926141246.48282-1-francesco@dolcini.it
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240926141246.48282-1-francesco@dolcini.it
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>