drm/omap: fix race with error_irq

drm/omap: fix race with error_irq

omapdrm tries to avoid error floods by unregistering the error irq when
an error happens, and then registering the error irq again later.
However, the code is racy, as it sometimes tries to unregister the error
irq when it's already unregistered, leading to WARN().

Also, the code only registers the error irq again when something is done
on that particular output, i.e. if only TV is used to flip the buffers,
and LCD is showing a same buffer, an error on LCD will cause the LCD
error irq to be unregistered and never registered again.

To fix this, let's keep the error irqs always enabled and trust the
DRM_ERROR_RATELIMITED to limit the flood.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>