drm/ssd130x: Remove hardcoded bits-per-pixel in ssd130x_buf_alloc()

The driver only supports OLED controllers that have a native DRM_FORMAT_C1
pixel format and that is why it has harcoded a division of the width by 8.

But the driver might be extended to support devices that have a different
pixel format. So it's better to use the struct drm_format_info helpers to
compute the size of the buffer, used to store the pixels in native format.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230609170941.1150941-6-javierm@redhat.com