drm/ssd130x: Inline the ssd130x_buf_{alloc, free}() function helpers

There is only a single caller for both helper functions and these don't do
much other than allocate and free two buffers, so let's just inline them.

Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230726105433.389740-1-javierm@redhat.com