drm/mipi-dbi: Support custom pipelines with drm_mipi_dbi_dev_init()

drm/mipi-dbi: Support custom pipelines with drm_mipi_dbi_dev_init()

Initialize the mipi-dbi device with drm_mipi_dbi_dev_init() without
creating a modesetting pipeline. Will allow for mipi-dbi drivers
without simple-display helpers.

As the new helper is a DRM function, add the drm_ prefix. Mipi-dbi
interfaces currently lack this.

v3:
- document tx_buf_size parameter (David)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: David Lechner <david@lechnology.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Link: https://patch.msgid.link/20260319160110.109610-3-tzimmermann@suse.de