drm/ssd130x: Use new regmap bulk write support to drop custom bus

Data writes for the ssd130x 4-wire SPI protocol need special handling, due
the Data/Command control (D/C) pin having to be toggled prior to the write.

The regmap API only allowed drivers to provide .reg_{read,write} callbacks
to do per register read/write, but didn't provide a way for drivers to do
the same for bulk read/writes.

For this reason, a custom regmap bus was used by the driver just to define
a bulk write callback that implements the D/C pin toggling. But the regmap
API has been extended to support defining bulk read/write handlers, so the
custom regmap bus is not needed anymore and could just be dropped.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220618174338.2253021-1-javierm@redhat.com