drm/bridge: tc358767: Simplify AUX data write

Simplify AUX data write by dropping index arithmetic and shifting and
replacing it with a call to a helper function that does two things:

    1. Copies user-provided data into a write buffer
    2. Transfers contents of the write buffer to up to 4 32-bit
       registers on the chip

Note that separate data endianness fix:

    tmp = (tmp << 8) | buf[i];

that was reserved for DP_AUX_I2C_WRITE looks really strange, since it
will place data differently depending on the passed user-data
size. E.g. for a write of 1 byte, data transferred to the chip would
look like:

[byte0] [dummy1] [dummy2] [dummy3]

whereas for a write of 4 bytes we'd get:

[byte3] [byte2] [byte1] [byte0]

Since there's no indication in the datasheet that I2C write buffer
should be treated differently than AUX write buffer and no comment in
the original code explaining why it was done this way, that special
I2C write buffer transformation was dropped in this patch.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Andrzej Hajda <a.hajda@samsung.com>
Cc: Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Andrey Gusakov <andrey.gusakov@cogentembedded.com>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Cory Tusar <cory.tusar@zii.aero>
Cc: Chris Healy <cphealy@gmail.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190619052716.16831-8-andrew.smirnov@gmail.com