drm/st7586: Use regular atomic helpers; drop simple-display helpers

drm/st7586: Use regular atomic helpers; drop simple-display helpers

Replace simple-display helpers with regular atomic helpers. Store the
pipeline elements in struct st7586_device and initialize them as part
of probing the device. Use mipi-dbi's existing helpers and initializer
macros where possible.

Effectively open-codes the modesetting code in the initializer helpers
of mipi-dbi and simple-display. St7586 requires custom helpers for
various pipeline elements, and non-freeing cleanup of the pipeline.

v3:
- return early in st7586_plane_helper_atomic_update (David)
v2:
- fix connector initialization

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: David Lechner <david@lechnology.com>
Tested-by: David Lechner <david@lechnology.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Link: https://patch.msgid.link/20260319160110.109610-12-tzimmermann@suse.de