drm/mcde: Provide vblank handling unconditionally

At the moment, vblank handling is only enabled together with
TE synchronization. However, the vblank IRQ is also working with
on displays without TE synchronization (e.g. DSI video mode panels).
It seems like the vblank IRQ is actually generated by the
MCDE hardware for the channel.

Therefore, the vblank handling should be working correctly in
all the cases and we can enable it unconditionally.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191106165835.2863-2-stephan@gerhold.net