drm/bridge: tc358768: Add support for long command tx via video buffer

drm/bridge: tc358768: Add support for long command tx via video buffer

TC358768 has two ways to send DSI commands: 1) buffer the payload data
into registers (DSICMD_WDx), which supports up to 8 bytes of payload, 2)
buffer the payload data into the video buffer, which supports up to 1024
bytes of payload.

The driver currently supports method 1).

Add support for transmitting long DSI commands (more than 8 bytes, up to
1024 bytes) using the video buffer. This mode can only be used before
the actual video transmission is enabled, i.e. the initial configuration.

Original version from Parth Pancholi <parth.pancholi@toradex.com>

Tested-by: João Paulo Gonçalves <joao.goncalves@toradex.com> # Toradex Verdin AM62
Reviewed-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Link: https://patch.msgid.link/20260311-tc358768-v2-7-e75a99131bd5@ideasonboard.com
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>