drm/ssd130x: Don't allocate buffers on each plane update

The resolutions for these panels are fixed and defined in the Device Tree,
so there's no point to allocate the buffers on each plane update and that
can just be done once.

Let's do the allocation and free on the encoder enable and disable helpers
since that's where others initialization and teardown operations are done.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230609170941.1150941-5-javierm@redhat.com