drm/ssd130x: Store the HW buffer in the driver-private CRTC state

The commit 45b58669 ("drm/ssd130x: Allocate buffer in the plane's
.atomic_check() callback") moved the allocation of the intermediate and
HW buffers from the encoder's .atomic_enable callback, to the plane's
.atomic_check callback.

This was suggested by Maxime Ripard, because drivers aren't allowed to
fail after the drm_atomic_helper_swap_state() function has been called.

And the encoder's .atomic_enable happens after the new atomic state has
been swapped, so allocations (that can fail) shouldn't be done there.

But the HW buffer isn't really tied to the plane's state. It has a fixed
size that only depends on the (also fixed) display resolution defined in
the Device Tree Blob.

That buffer can be considered part of the CRTC state, and for this reason
makes more sense to do its allocation in the CRTC .atomic_check callback.

The other allocated buffer (used to store a conversion from the emulated
XR24 format to the native R1 format) is part of the plane's state, since
it will be optional once the driver supports R1 and allows user-space to
set that pixel format.

So let's keep the allocation for it in the plane's .atomic_check callback,
this can't be moved to the CRTC's .atomic_check because changing a format
does not trigger a CRTC mode set.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Closes: https://lore.kernel.org/dri-devel/CAMuHMdWv_QSatDgihr8=2SXHhvp=icNxumZcZOPwT9Q_QiogNQ@mail.gmail.com/


Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230913052938.1114651-1-javierm@redhat.com