drm/gma500: Remove in_atomic() usage.

The driver is using msleep() if it is safe to use based on in_atomic().
This is not needed this macro is only used from
i2c_algorithm::master_xfer() which is always invoked from preemptible
context.

Remove in_atomic() because it is superfluous. Remove wait_for_atomic()
because it has no users.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210208233119.391103-2-bigeasy@linutronix.de